Lanthanide and Actinide Chemistry (Inorganic Chemistry: A Textbook Series)

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Lanthanide and Actinide
Chemistry
Simon Cotton
Uppingham School, Uppingham, Rutland, UK

Lanthanide and Actinide Chemistry

Inorganic Chemistry
A Wiley Series of Advanced Textbooks
Editorial Board

Derek Woollins, University of St. Andrews, UK
Bob Crabtree, Yale University, USA
David Atwood, University of Kentucky, USA
Gerd Meyer, University of Hannover, Germany
Previously Published Books In this Series

Chemical Bonds: A Dialog
Author: J. K. Burdett
Bioinorganic Chemistry: Inorganic Elements in the Chemistry
of Life – An Introduction and Guide
Author: W. Kaim
Synthesis of Organometallic Compounds: A Practical Guide
Edited by: S. Komiya
Main Group Chemistry Second Edition
Author: A. G. Massey
Inorganic Structural Chemistry
Author: U. Muller
Stereochemistry of Coordination Compounds
Author: A. Von Zelewsky
Lanthanide and Actinide Chemistry
Author: S. A. Cotton

Lanthanide and Actinide
Chemistry
Simon Cotton
Uppingham School, Uppingham, Rutland, UK

C 2006
Copyright 


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Library of Congress Cataloging-in-Publication Data
Cotton, Simon, Dr.
Lanthanide and actinide chemistry / Simon Cotton.
p. cm. – (Inorganic chemistry)
Includes bibliographical references and index.
ISBN-13: 978-0-470-01005-1 (acid-free paper)
ISBN-10: 0-470-01005-3 (acid-free paper)
ISBN-13: 978-0-470-01006-8 (pbk. : acid-free paper)
ISBN-10: 0-470-01006-1 (pbk. : acid-free paper)
1. Rare earth metals. 2. Actinide elements. I. Title.
II. Inorganic chemistry (John Wiley & Sons)
QD172.R2C68 2006
546 .41—dc22
2005025297
British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British Library
ISBN-13 9-78-0-470-01005-1 (Cloth) 9-78-0-470-01006-8 (Paper)
ISBN-10 0-470-01005-3 (Cloth)
0-470-01006-1 (Paper)
Typeset in 10/12pt Times by TechBooks, New Delhi, India
Printed and bound in Great Britain by Antony Rowe, Chippenham, Wiltshire
This book is printed on acid-free paper responsibly manufactured from sustainable forestry
in which at least two trees are planted for each one used for paper production.

In memory of Ray and Derek Cotton, my parents.
Remember that it was of your parents you were born;
how can you repay what they have given to you?
(Ecclesiasticus 7.28 RSV)
also in memory of Marı́a de los Ángeles Santiago Hernández,
a lovely lady and devout Catholic, who died far too young.
and to Lisa.

Dr Simon Cotton obtained his PhD at Imperial College London. After postdoctoral research and teaching appointments at Queen Mary College, London, and the University of
East Anglia, he has taught chemistry in several different schools, and has been at Uppingham School since 1996. From 1984 until 1997, he was Editor of Lanthanide and Actinide
Compounds for the Dictionary of Organometallic Compounds and the Dictionary of Inorganic Compounds. He authored the account of Lanthanide Coordination Chemistry for the
2nd edition of Comprehensive Coordination Chemistry (Pergamon) as well as the accounts
of Lanthanide Inorganic and Coordination Chemistry for both the 1st and 2nd editions of
the Encyclopedia of Inorganic Chemistry (Wiley).
His previous books are:
S.A. Cotton and F.A. Hart, “The Heavy Transition Elements”, Macmillan, 1975.
D.J. Cardin, S.A. Cotton, M. Green and J.A. Labinger, “Organometallic Compounds of the
Lanthanides, Actinides and Early Transition Metals”, Chapman and Hall, 1985.
S.A. Cotton, “Lanthanides and Actinides”, Macmillan, 1991.
S.A. Cotton, “Chemistry of Precious Metals”, Blackie, 1997.

Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 Introduction to the Lanthanides . . . . . . . . . . . .
1.1 Introduction . . . . . . . . . . . . . . . . . . . .
1.2 Characteristics of the Lanthanides . . . . . . . . .
1.3 The Occurrence and Abundance of the Lanthanides .
1.4 Lanthanide Ores . . . . . . . . . . . . . . . . . .
1.5 Extracting and Separating the Lanthanides . . . . .
1.5.1 Extraction . . . . . . . . . . . . . . . . .
1.5.2 Separating the Lanthanides . . . . . . . . .
1.6 The Position of the Lanthanides in the Periodic Table
1.7 The Lanthanide Contraction . . . . . . . . . . . .

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xv

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1
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2
2
3
3
3
4
6
7

2 The Lanthanides – Principles and Energetics . . . . . . . . . . . . . . . . . . . . . .
2.1 Electron Configurations of the Lanthanides and f Orbitals . . . . . . . . . . . . . . .
2.2 What do f Orbitals Look Like? . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 How f Orbitals affect Properties of the Lanthanides . . . . . . . . . . . . . . . . . .
2.4 The Lanthanide Contraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5 Electron Configurations of the Lanthanide Elements and of Common Ions . . . . . . .
2.6 Patterns in Ionization Energies . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.7 Atomic and Ionic Radii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.8 Patterns in Hydration Energies (Enthalpies) for the Lanthanide Ions . . . . . . . . . .
2.9 Enthalpy Changes for the Formation of Simple Lanthanide Compounds . . . . . . . .
2.9.1 Stability of Tetrahalides . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.9.2 Stability of Dihalides . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.9.3 Stability of Aqua Ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.10 Patterns in Redox Potentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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14
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19

3 The Lanthanide Elements and Simple Binary Compounds . . . . . . . . . . . . . . . .
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 The Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1 Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.2 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.3 Alloys and Uses of the Metals . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 Binary Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1 Trihalides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.2 Tetrahalides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.3 Dihalides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.4 Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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x

Contents
Borides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Carbides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Nitrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hydrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sulfides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

31
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31
31
32

4 Coordination Chemistry of the Lanthanides . . . . . . . . . . . . . . . . . . . . . . .
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Stability of Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.1 The Aqua Ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.2 Hydrated Salts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.3 Other O-Donors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.4 Complexes of β-Diketonates . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.5 Lewis Base Adducts of β-Diketonate Complexes . . . . . . . . . . . . . . . .
4.3.6 Nitrate Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.7 Crown Ether Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.8 Complexes of EDTA and Related Ligands . . . . . . . . . . . . . . . . . . .
4.3.9 Complexes of N-Donors . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.10 Complexes of Porphyrins and Related Systems . . . . . . . . . . . . . . . . .
4.3.11 Halide Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.12 Complexes of S-Donors . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4 Alkoxides, Alkylamides and Related Substances . . . . . . . . . . . . . . . . . . . .
4.4.1 Alkylamides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.2 Alkoxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.3 Thiolates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5 Coordination Numbers in Lanthanide Complexes . . . . . . . . . . . . . . . . . . .
4.5.1 General Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5.2 Examples of the Coordination Numbers . . . . . . . . . . . . . . . . . . . .
4.5.3 The Lanthanide Contraction and Coordination Numbers . . . . . . . . . . . .
4.5.4 Formulae and Coordination Numbers . . . . . . . . . . . . . . . . . . . . .
4.6 The Coordination Chemistry of the +2 and +4 States . . . . . . . . . . . . . . . . .
4.6.1 The (+2) State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6.2 The (+4) State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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5 Electronic and Magnetic Properties of the Lanthanides . . . . . . . . . . . . . . . . .
5.1 Magnetic and Spectroscopic Properties of the Ln3+ Ions . . . . . . . . . . . . . . . .
5.2 Magnetic Properties of the Ln3+ Ions . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1 Adiabatic Demagnetization . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 Energy Level Diagrams for the Lanthanide Ions, and their
Electronic Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.1 Electronic Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.2 Hypersensitive Transitions . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4 Luminescence Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.1 Quenching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.2 Antenna Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.3 Applications of Luminescence to Sensory Probes . . . . . . . . . . . . . . . .
5.4.4 Fluorescence and TV . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.5 Lighting Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3.4
3.5
3.6
3.7
3.8

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Contents

xi

5.4.6 Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.7 Euro Banknotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5 NMR Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.1 β-Diketonates as NMR Shift Reagents . . . . . . . . . . . . . . . . . . . . .
5.5.2 Magnetic Resonance Imaging (MRI) . . . . . . . . . . . . . . . . . . . . . .
5.5.3 What Makes a Good MRI Agent? . . . . . . . . . . . . . . . . . . . . . . .
5.5.4 Texaphyrins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6 Electron Paramagnetic Resonance Spectroscopy . . . . . . . . . . . . . . . . . . . .
5.7 Lanthanides as Probes in Biological Systems . . . . . . . . . . . . . . . . . . . . .

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6 Organometallic Chemistry of the Lanthanides . . . . . . . . . . . . . . . . . . . . . .
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2 The +3 Oxidation State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.1 Alkyls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.2 Aryls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3 Cyclopentadienyls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3.1 Compounds of the Unsubstituted Cyclopentadienyl Ligand
(C5 H5 = Cp; C5 Me5 = Cp*) . . . . . . . . . . . . . . . . . . . . . . . . .
6.3.2 Compounds [LnCp*3 ] (Cp* = Pentamethylcyclopentadienyl) . . . . . . . . . .
6.3.3 Bis(cyclopentadienyl) Alkyls and Aryls LnCp2 R . . . . . . . . . . . . . . . .
6.3.4 Bis(pentamethylcyclopentadienyl) Alkyls . . . . . . . . . . . . . . . . . . .
6.3.5 Hydride Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4 Cyclooctatetraene Dianion Complexes . . . . . . . . . . . . . . . . . . . . . . . .
6.5 The +2 State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.5.1 Alkyls and Aryls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.5.2 Cyclopentadienyls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.5.3 Other Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.6 The +4 State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.7 Metal–Arene Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.8 Carbonyls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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102

7 The Misfits: Scandium, Yttrium, and Promethium . . . . . . . . . . . . . . . . . . . .
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2 Scandium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2.1 Binary Compounds of Scandium . . . . . . . . . . . . . . . . . . . . . . .
7.3 Coordination Compounds of Scandium . . . . . . . . . . . . . . . . . . . . . . . .
7.3.1 The Aqua Ion and Hydrated Salts . . . . . . . . . . . . . . . . . . . . . . .
7.3.2 Other Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.3 Alkoxides and Alkylamides . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.4 Patterns in Coordination Number . . . . . . . . . . . . . . . . . . . . . . .
7.4 Organometallic Compounds of Scandium . . . . . . . . . . . . . . . . . . . . . . .
7.5 Yttrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.6 Promethium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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8 The Lanthanides and Scandium in Organic Chemistry . . . . . . . . . . . . . . . . . .
8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2 Cerium(IV) Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2.1 Oxidation of Aromatics . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2.2 Oxidation of Alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2.3 Oxidation of Alcohols . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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xii

Contents
8.3 Samarium(II) Iodide, SmI2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3.1 Reduction of Halogen Compounds . . . . . . . . . . . . . . . . . . . . . . .
8.3.2 Reduction of α-Heterosubstituted Ketones . . . . . . . . . . . . . . . . . . .
8.3.3 Reductions of Carbonyl Groups . . . . . . . . . . . . . . . . . . . . . . . .
8.3.4 Barbier Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3.5 Reformatsky Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.4 Lanthanide β-Diketonates as Diels–Alder Catalysts . . . . . . . . . . . . . . . . . .
8.4.1 Hetero-Diels–Alder Reactions . . . . . . . . . . . . . . . . . . . . . . . . .
8.5 Cerium(III) Chloride and Organocerium Compounds . . . . . . . . . . . . . . . . .
8.6 Cerium(III) Chloride and Metal Hydrides . . . . . . . . . . . . . . . . . . . . . . .
8.7 Scandium Triflate and Lanthanide Triflates . . . . . . . . . . . . . . . . . . . . . .
8.7.1 Friedel–Crafts Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.7.2 Diels–Alder Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.7.3 Mannich Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.7.4 Imino-Diels–Alder Reactions . . . . . . . . . . . . . . . . . . . . . . . . .
8.8 Alkoxides and Aryloxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.9 Lanthanide Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.10 Organometallics and Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . .

124
124
124
124
126
127
127
128
128
130
131
131
132
132
133
134
135
136

9 Introduction to the Actinides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.1 Introduction and Occurrence of the Actinides . . . . . . . . . . . . . . . . . . . . .
9.2 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.3 Extraction of Th, Pa, and U . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.3.1 Extraction of Thorium . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.3.2 Extraction of Protactinium . . . . . . . . . . . . . . . . . . . . . . . . . .
9.3.3 Extraction and Purification of Uranium . . . . . . . . . . . . . . . . . . . .
9.4 Uranium Isotope Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.4.1 Gaseous Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.4.2 Gas Centrifuge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.4.3 Electromagnetic Separation . . . . . . . . . . . . . . . . . . . . . . . . . .
9.4.4 Laser Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.5 Characteristics of the Actinides . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.6 Reduction Potentials of the Actinides . . . . . . . . . . . . . . . . . . . . . . . . .
9.7 Relativistic Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

145
145
145
147
147
148
148
148
148
148
149
149
149
150
152

10 Binary Compounds of the Actinides . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.2 Halides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.2.1 Syntheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.2.2 Structure Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.3 Thorium Halides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.4 Uranium Halides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.4.1 Uranium(VI) Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.4.2 Uranium(V) Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.4.3 Uranium (IV) Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.4.4 Uranium(III) Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.4.5 Uranium Hexafluoride and Isotope Separation . . . . . . . . . . . . . . . . .

155
155
155
156
158
159
160
161
161
161
163
163

Contents

xiii
The Actinide Halides (Ac–Am) excluding U and Th . . . . . . . . . . . . . . . . .
10.5.1 Actinium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.5.2 Protactinium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.5.3 Neptunium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.5.4 Plutonium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.5.5 Americium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Halides of the Heavier Transactinides . . . . . . . . . . . . . . . . . . . . . . . .
10.6.1 Curium(III) Chloride . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.6.2 Californium(III) Chloride, Californium(III) Iodide, and Californium(II) Iodide .
10.6.3 Einsteinium(III) Chloride . . . . . . . . . . . . . . . . . . . . . . . . . .
Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.7.1 Thorium Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.7.2 Uranium Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Uranium Hydride UH3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Oxyhalides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

165
165
165
166
166
167
168
168
168
168
169
169
169
170
170

11 Coordination Chemistry of the Actinides . . . . . . . . . . . . . . . . . . . . . . . . .
11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.2 General Patterns in the Coordination Chemistry of the Actinides . . . . . . . . . . .
11.3 Coordination Numbers in Actinide Complexes . . . . . . . . . . . . . . . . . . . .
11.4 Types of Complex Formed . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.5 Uranium and Thorium Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . .
11.5.1 Uranyl Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.5.2 Coordination Numbers and Geometries in Uranyl Complexes . . . . . . . .
11.5.3 Some Other Complexes . . . . . . . . . . . . . . . . . . . . . . . . . .
11.5.4 Uranyl Nitrate and its Complexes; their Role in Processing Nuclear Waste . .
11.5.5 Nuclear Waste Processing . . . . . . . . . . . . . . . . . . . . . . . . .
11.6 Complexes of the Actinide(IV) Nitrates and Halides . . . . . . . . . . . . . . . . .
11.6.1 Thorium Nitrate Complexes . . . . . . . . . . . . . . . . . . . . . . . .
11.6.2 Uranium(IV) Nitrate Complexes . . . . . . . . . . . . . . . . . . . . . .
11.6.3 Complexes of the Actinide(IV) Halides . . . . . . . . . . . . . . . . . . .
11.7 Thiocyanates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.8 Amides, Alkoxides and Thiolates . . . . . . . . . . . . . . . . . . . . . . . . . .
11.8.1 Amide Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.8.2 Alkoxides and Aryloxides . . . . . . . . . . . . . . . . . . . . . . . . .
11.9 Chemistry of Actinium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.10 Chemistry of Protactinium . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.11 Chemistry of Neptunium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.11.1 Complexes of Neptunium . . . . . . . . . . . . . . . . . . . . . . . . .
11.12 Chemistry of Plutonium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.12.1 Aqueous Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.12.2 The Stability of the Oxidation States of Plutonium . . . . . . . . . . . . . .
11.12.3 Coordination Chemistry of Plutonium . . . . . . . . . . . . . . . . . . .
11.12.4 Plutonium in the Environment . . . . . . . . . . . . . . . . . . . . . . .
11.13 Chemistry of Americium and Subsequent Actinides . . . . . . . . . . . . . . . . .
11.13.1 Potentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.14 Chemistry of the Later Actinides . . . . . . . . . . . . . . . . . . . . . . . . . .

173
173
173
174
174
175
175
177
178
179
179
180
180
180
181
183
184
184
185
186
187
188
188
189
189
190
191
193
195
195
196

10.5

10.6

10.7

10.8
10.9

xiv

Contents

12 Electronic and Magnetic Properties of the Actinides . . . . . . . . . . . . . . . . . . .
12.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.2 Absorption Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.2.1 Uranium(VI) – UO2 2+ – f 0 . . . . . . . . . . . . . . . . . . . . . . . . .
12.2.2 Uranium(V) – f 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.2.3 Uranium(IV) – f 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.2.4 Spectra of the Later Actinides . . . . . . . . . . . . . . . . . . . . . . .
12.3 Magnetic Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

201
201
202
202
203
204
206
207

13 Organometallic Chemistry of the Actinides . . . . . . . . . . . . . . . . . . . . . . .
13.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.2 Simple σ-Bonded Organometallics . . . . . . . . . . . . . . . . . . . . . . . . .
13.3 Cyclopentadienyls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.3.1 Oxidation State (VI) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.3.2 Oxidation State (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.3.3 Oxidation State (IV) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.3.4 Oxidation State (III) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.4 Compounds of the Pentamethylcyclopentadienyl Ligand (C5 Me5 = Cp*) . . . . . . .
13.4.1 Oxidation State(IV) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.4.2 Cationic Species and Catalysts . . . . . . . . . . . . . . . . . . . . . . .
13.4.3 Hydrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.4.4 Oxidation State (III) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.5 Tris(pentamethylcyclopentadienyl) Systems . . . . . . . . . . . . . . . . . . . . .
13.6 Other Metallacycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.7 Cyclooctatetraene Dianion Compounds . . . . . . . . . . . . . . . . . . . . . . .
13.8 Arene Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.9 Carbonyls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

209
209
209
211
211
211
211
214
215
215
216
217
218
219
219
219
221
222

14 Synthesis of the Transactinides and their Chemistry . . . . . . . . . . . . . . . . . . .
14.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.2 Finding New Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.3 Synthesis of the Transactinides . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.4 Naming the Transactinides . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.5 Predicting Electronic Arrangements . . . . . . . . . . . . . . . . . . . . . . . . .
14.6 Identifying the Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.7 Predicting Chemistry of the Transactinides . . . . . . . . . . . . . . . . . . . . .
14.8 What is known about the Chemistry of the Transactinides . . . . . . . . . . . . . .
14.8.1 Element 104 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.8.2 Element 105 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.8.3 Element 106 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.8.4 Element 107 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.8.5 Element 108 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.9 And the Future? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

225
225
226
226
229
230
230
233
234
234
234
234
235
235
236

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

237

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

253

Preface
This book is aimed at providing a sound introduction to the chemistry of the lanthanides,
actinides and transactinides to undergraduate students. I hope that it will also be of value
to teachers of these courses. Whilst not being anything resembling a comprehensive monograph, it does attempt to give a factual basis to the area, and the reader can use a fairly
comprehensive bibliography to range further.
Since I wrote a previous book in this area (1991), the reader may wonder why on earth I
have bothered again. The world of f-block chemistry has moved on. It is one of active and
important research, with names like Bünzli, Evans, Ephritikhine, Lappert, Marks and Parker
familiar world-wide (I am conscious of names omitted). Not only have several more elements
been synthesized (and claims made for others), but lanthanides and their compounds are
routinely employed in many areas of synthetic organic chemistry; gadolinium compounds
find routine application in MRI scans; and there are other spectroscopic applications, notably
in luminescence. Whilst some areas are hardly changed, at this level at least (e.g. actinide
magnetism and spectroscopy), a lot more compounds have been described, accounting for
the length of the chapters on coordination and organometallic chemistry. I have tried to
spell out the energetics of lanthanide chemistry in more detail, whilst I have provided some
end-of-chapter questions, of variable difficulty, which may prove useful for tutorials. I have
supplied most, but not all, of the answers to these (my answers, which are not always
definitive).
It is a pleasure to thank all those who have contributed to the book: Professor Derek
Woollins, for much encouragement at different stages of the project; Professor James
Anderson, for many valuable comments on Chapter 8; Martyn Berry, who supplied valuable
comment on early versions of several chapters; to Professors Michel Ephritikhine, Allan
White and Jack Harrowfield, and Dr J.A.G. Williams, and many others, for exchanging
e-mails, correspondence and ideas. I’m very grateful to Dr Mary P. Neu for much information on plutonium. The staff of the Libraries of the Chemistry Department of Cambridge
University and of the Royal Society of Chemistry, as well as the British Library, have
been quite indispensable in helping with access to the primary literature. I would also wish
to thank a number of friends – once again Dr Alan Hart, who got me interested in lanthanides in the first place; Professor James Anderson (again), Dr Andrew Platt, Dr John
Fawcett, and Professor Paul Raithby, for continued research collaboration and obtaining
spectra and structures from unpromising crystals, so that I have kept a toe-hold in the area.
Over the last 8 years, a number of Uppingham 6th form students have contributed to my
efforts in lanthanide coordination chemistry – John Bower, Oliver Noy, Rachel How, Vilius
Franckevicius, Leon Catallo, Franz Niecknig, Victoria Fisher, Alex Tait and Joanna Harris.
Finally, thanks are most certainly due to Dom Paul-Emmanuel Clénet and the Benedictine
community of the Abbey of Bec, for continued hospitality during several Augusts when I
have been compiling the book.
Simon Cotton

1 Introduction to the Lanthanides
By the end of this chapter you should be able to:

r understand that lanthanides differ in their properties from the s- and d-block metals;
r recall characteristic properties of these elements;
r appreciate reasons for their positioning in the Periodic Table;
r understand how the size of the lanthanide ions affects certain properties and how this can
be used in the extraction and separation of the elements;

r understand how to obtain pure samples of individual Ln3+ ions.
1.1 Introduction
Lanthanide chemistry started in Scandinavia. In 1794 Johann Gadolin succeeded in obtaining an ‘earth’(oxide) from a black mineral subsequently known as gadolinite; he called
the earth yttria. Soon afterwards, M.H. Klaproth, J.J. Berzelius and W. Hisinger obtained
ceria, another earth, from cerite. However, it was not until 1839–1843 that the Swede C.G.
Mosander first separated these earths into their component oxides; thus ceria was resolved
into the oxides of cerium and lanthanum and a mixed oxide ‘didymia’ (a mixture of the
oxides of the metals from Pr through Gd). The original yttria was similarly separated into
substances called erbia, terbia, and yttria (though some 40 years later, the first two names
were to be reversed!). This kind of confusion was made worse by the fact that the newly
discovered means of spectroscopic analysis permitted misidentifications, so that around 70
‘new’ elements were erroneously claimed in the course of the century.
Nor was Mendeleev’s revolutionary Periodic Table a help. When he first published his
Periodic Table in 1869, he was able to include only lanthanum, cerium, didymium (now
known to have been a mixture of Pr and Nd), another mixture in the form of erbia, and
yttrium; unreliable information about atomic mass made correct positioning of these elements in the table difficult. Some had not yet been isolated as elements. There was no way
of predicting how many of these elements there would be until Henry Moseley (1887–1915)
analysed the X-ray spectra of elements and gave meaning to the concept of atomic number.
He showed that there were 15 elements from lanthanum to lutetium (which had only been
identified in 1907). The discovery of radioactive promethium had to wait until after World
War 2.
It was the pronounced similarity of the lanthanides to each other, especially each to its
neighbours (a consequence of their general adoption of the +3 oxidation state in aqueous
solution), that caused their classification and eventual separation to be an extremely difficult
undertaking.

Lanthanide and Actinide Chemistry S. Cotton


C 2006 John Wiley & Sons, Ltd.

2

Introduction to the Lanthanides
Subsequently it was not until the work of Bohr and of Moseley that it was known precisely
how many of these elements there were. Most current versions of the Periodic Table place
lanthanum under scandium and yttrium.

1.2 Characteristics of the Lanthanides
The lanthanides exhibit a number of features in their chemistry that differentiate them from
the d-block metals. The reactivity of the elements is greater than that of the transition metals,
akin to the Group II metals:
1. A very wide range of coordination numbers (generally 6–12, but numbers of 2, 3 or 4
are known).
2. Coordination geometries are determined by ligand steric factors rather than crystal field
effects.
3. They form labile ‘ionic’ complexes that undergo facile exchange of ligand.
4. The 4f orbitals in the Ln3+ ion do not participate directly in bonding, being well shielded
by the 5s2 and 5p6 orbitals. Their spectroscopic and magnetic properties are thus largely
uninfluenced by the ligand.
5. Small crystal-field splittings and very sharp electronic spectra in comparison with the
d-block metals.
6. They prefer anionic ligands with donor atoms of rather high electronegativity (e.g.
O, F).
7. They readily form hydrated complexes (on account of the high hydration energy of the
small Ln3+ ion) and this can cause uncertainty in assigning coordination numbers.
8. Insoluble hydroxides precipitate at neutral pH unless complexing agents are present.
9. The chemistry is largely that of one (3+) oxidation state (certainly in aqueous solution).
10. They do not form Ln=O or Ln≡N multiple bonds of the type known for many transition
metals and certain actinides.
11. Unlike the transition metals, they do not form stable carbonyls and have (virtually) no
chemistry in the 0 oxidation state.

1.3 The Occurrence and Abundance of the Lanthanides
Table 1.1 presents the abundance of the lanthanides in the earth’s crust and in the solar
system as a whole. (Although not in the same units, the values in each list are internally
consistent.)
Two patterns emerge from these data. First, that the lighter lanthanides are more abundant
than the heavier ones; secondly, that the elements with even atomic number are more
abundant than those with odd atomic number. Overall, cerium, the most abundant lanthanide
Table 1.1 Abundance of the lanthanides

Crust (ppm)
Solar System
(with respect to
107 atoms Si)

La

Ce

Pr

Nd

Pm

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Y

35
4.5

66
1.2

9.1
1.7

40
8.5

0.0
0.0

7
2.5

2.1
1.0

6.1
3.3

1.2
0.6

4.5
3.9

1.3
0.9

3.5
2.5

0.5
0.4

3.1
2.4

0.8
0.4

31
40.0

Extracting and Separating the Lanthanides

3

on earth, has a similar crustal concentration to the lighter Ni and Cu, whilst even Tm and
Lu, the rarest lanthanides, are more abundant than Bi, Ag or the platinum metals.
The abundances are a consequence of how the elements were synthesized by atomic fusion
in the cores of stars with heavy elements only made in supernovae. Synthesis of heavier
nuclei requires higher temperature and pressures and so gets progressively harder as the
atomic number increases. The odd/even alternation (often referred to as the Oddo–Harkins
rule) is again general, and reflects the facts that elements with odd mass numbers have larger
nuclear capture cross sections and are more likely to take up another neutron, so elements
with odd atomic number (and hence odd mass number) are less common than those with
even mass number. Even-atomic-number nuclei are more stable when formed.

1.4 Lanthanide Ores
Principal sources (Table 1.2) are the following:
Bastnasite LnFCO3 ; Monazite (Ln, Th)PO4 (richer in earlier lanthanides); Xenotime
(Y, Ln)PO4 (richer in later lanthanides). In addition to these, there are Chinese rare earth
reserves which amount to over 70% of the known world total, mainly in the form of the
ionic ores from southern provinces. These Chinese ion-absorption ores, weathered granites
with lanthanides adsorbed onto the surface of aluminium silicates, are in some cases low in
cerium and rich in the heavier lanthanides (Longnan) whilst the Xunwu deposits are rich in
the lighter metals; the small particle size makes them easy to mine. The Chinese ores have
made them a leading player in lanthanide chemistry.
Table 1.2 Typical abundance of the lanthanides in oresa
%

La

Monazite
Bastnasite
Xenotime

20
43
4.5 16
0
33.2 49.1 4.3 12
0
0.5 5
0.7 2.2 0

a Bold

Ce

Pr

Nd

Pm Sm Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb Lu

Y

3
0.1 1.5
0.05
0.6
0.05 0.2 0.02 0.1 0.02 2.5
0.8 0.12 0.17 160
310
50
35
8
6
1
0.1
1.9 0.2 4
1
8.6 2
5.4 0.9 6.2 0.4 60.0

values are in ppm.

1.5 Extracting and Separating the Lanthanides
These two processes are not necessarily coterminous. Whilst electronic, optical and magnetic applications require individual pure lanthanides, the greatest quantity of lanthanides
is used as mixtures, e.g. in mischmetal or oxide catalysts.

1.5.1 Extraction
After initial concentration by crushing, grinding and froth flotation, bastnasite is treated with
10% HCl to remove calcite, by which time the mixture contains around 70% lanthanide
oxides. This is roasted to oxidize the cerium content to CeIV ; on further extraction with
HCl, the Ce remains as CeO2 , whilst the lanthanides in the (+3) state dissolve as a solution
of the chlorides.
Monazite is usually treated with NaOH at 150 ◦ C to remove phosphate as Na3 PO4 , leaving
a mixture of the hydrated oxides, which are dissolved in boiling HCl at pH 3.5, separating the
lanthanides from insoluble ThO2 . Sulfuric acid can also be used to dissolve the lanthanides.

4

Introduction to the Lanthanides

1.5.2 Separating the Lanthanides
These can be divided into four types: chemical separations, fractional crystallization, ionexchange methods and solvent extraction. Of these, only the last-named is used on a commercial scale (apart from initial separation of cerium). Chemical separations rely on using
stabilities of unusual oxidation states; thus Eu2+ is the only ion in that oxidation state formed
on reduction by zinc amalgam and can then be precipitated as EuSO4 (note the similarity
with heavier Group 2 metals). Repeated (and tedious) fractional crystallization, which made
use of slight solubility differences between the salts of neighbouring lanthanides, such as
the bromates Ln(BrO3 )3 .9H2 O, ethyl sulfates and double nitrates, were once the only possible way of obtaining pure lanthanides, as with the 15 000 recrystallizations carried out
by the American C. James to get pure thulium bromate (1911) (Figure 1.1 indicates the
principle of this method).
original
Legend
solution
 = dissolve in H2O
= evaporate to crystals
= mother liquor
A1
= crystals
= combine
B1
A2

D1
ble
olu
t s uent
as
Le nstit
co

E1

B2
C2

D2
E2

A3
B3

C3
D3

A4
B4

C4

A5
B5

Mo
co st so
ns lu
titu ble
en
t

C1

Middle fraction

Figure 1.1
Diagrammatic representation of the system of fractional crystallization used to separate salts of the
rare-earth elements (reproduced with permission from D.M. Yost, H. Russell and C.S. Garner, The
Rare Earth Elements and their Compounds, John Wiley, 1947.)

Ion-exchange chromatography is not of real commercial importance for large-scale production, but historically it was the method by which fast high-purity separation of the
lanthanides first became feasible. As radioactive lanthanide isotopes are important fission
products of the fission of 235 U and therefore need to be separated from uranium, and because the actinides after plutonium tend to resemble the lanthanides, the development of
the technique followed on the Manhattan project. It was found that if Ln3+ ions were adsorbed at the top of a cation-exchange resin, then treated with a complexing agent such as
buffered citric acid, then the cations tended to be eluted in reverse atomic number order
(Figure 1.2a); the anionic ligand binds most strongly to the heaviest (and smallest) cation,
which has the highest charge density. A disadvantage of this approach when scaled up to
high concentration is that the peaks tend to overlap (Figure 1.2b).
It was subsequently found that amine polycarboxylates such as EDTA4− gave stronger
complexes and much better separations. In practice, some Cu2+ ions (‘retainer’) are
added to prevent precipitation of either the free acid H4 EDTA or the lanthanide complex

Extracting and Separating the Lanthanides
105

5

Ho

Lu

Concentration – mg. oxide/ liter

Concentration – counts/ min.

Tb

Y
Tm
Dy

101
0

2500

150
Sm

Nd
Pr

0
0

Elution Time – min.
(a)

60

Volume of Eluate – liters
(b)

Figure 1.2
(a) Cation-exchange chromatography of lanthanides, (b) overlap of peaks at high concentration. (a) Tracer-scale elution
with 5% citrate at pH 3.20 (redrawn from B.H. Ketelle and G.E. Boyd, J. Am. Chem. Soc., 1947, 69, 2800). (b) Macro-scale
elution with 0.1% citrate at pH 5.30 (redrawn from F.H. Spedding, E.I. Fulmer, J.E. Powell, and T.A. Butler, J. Am. Chem.
c
Soc., 1950, 72, 2354). Reprinted with permission of the American Chemical Society 
1978.

HLn(EDTA).xH2 O on the resin. The major disadvantage of this method is that it is a slow
process for large-scale separations.
Solvent extraction has come to be used for the initial stage of the separation process, to
give material with up to 99.9% purity. In 1949, it was found that Ce4+ could readily be
separated from Ln3+ ions by extraction from a solution in nitric acid into tributyl phosphate
[(BuO)3 PO]. Subsequently the process was extended to separating the lanthanides, using a
non-polar organic solvent such as kerosene and an extractant such as (BuO)3 PO or bis
(2-ethylhexyl)phosphinic acid [[C4 H9 CH(C2 H5 )]2 P=O(OH)] to extract the lanthanides
from aqueous nitrate solutions. The heavier lanthanides form complexes which are more
soluble in the aqueous layer. After the two immiscible solvents have been agitated together
and separated, the organic layer is treated with acid and the lanthanide extracted. The solvent
is recycled and the aqueous layer put through further stages.
For a lanthanide LnA distributed between two phases, a distribution coefficent DA is
defined:
DA = [LnA in organic phase] / [LnA in aqueous phase]
For two lanthanides LnA and LnB in a mixture being separated, a separation factor βBA can
be defined, where
βBA = DA /DB
β is very close to unity for two adjacent lanthanides in the Periodic Table (obviously, the
larger β is, the better the separation).
In practice this process is run using an automated continuous counter-current circuit in
which the organic solvent flows in the opposite direction to the aqueous layer containing
the lanthanides. An equilibrium is set up between the lanthanide ions in the aqueous phase
and the organic layer, with there tending to be a relative enhancement of the concentration
of the heavier lanthanides in the organic layer. Because the separation between adjacent

6

Introduction to the Lanthanides

Back-extract
(water)

Scrub
5M HNO3

Back extraction
10 stages

Stripped
solvent
for
re-cycling

Scrub

Solvent
containing
Sm-Nd-HNO3

Aqueous
solution
Pr-La

Precipitate
and
re-dissolve

Concentrate
or
precipitate
and
re-dissolve
Solvent

Feed

Stripped
solvent

Aqueous
Sm, etc.

Scrub

Solvent

Feed

Back-extract
Scrub and Extraction

Solvent
Sm

Solvent
(Undiluted
tributyl phosphate)

Aqueous
solution
Sm-Nd

Back extract
Back
extraction

Feed
Saturated
neutral nitrate
solution of
La-Pr-Nd-Sm
Scrub and extraction
Perhaps 20 stages

Back
extraction

Scrub and extraction
Solvent
Pr

Stripped
solvent
Aqueous
Aqueous
Pr
Nd

Aqueous
La

Figure 1.3
Schematic diagram of lanthanon separation by solvent extraction. From R.J. Callow, The Rare Earth
Industry, Pergamon, 1966; reproduced by permission.

lanthanides in each exchange is relatively slight, over a thousand exchanges are used (see
Figure 1.3). This method affords lanthanides of purity up to the 99.9% purity level and is
thus well suited to large-scale separation, the products being suited to ordinary chemical
use. However, for electronic or spectroscopic use (‘phosphor grade’) 99.999% purity is
necessary, and currently ion-exchange is used for final purification to these levels. The
desired lanthanides are precipitated as the oxalate or hydroxide and converted into the
oxides (the standard starting material for many syntheses) by thermal decomposition.
Various other separation methods have been described, one recent one involving the use
of supercritical carbon dioxide at 40 ◦ C and 100 atm to convert the lanthanides into their
carbonates whilst the quadrivalent metals (e.g. Th and Ce) remain as their oxides.

1.6 The Position of the Lanthanides in the Periodic Table
As already mentioned, neither Mendeleev nor his successors could ‘place’ the lanthanides in
the Periodic Table. Not only was there no recognizable atomic theory until many years afterwards, but, more relevant to how groupings of elements were made in those days, there was
no comparable block of elements for making comparisons. The lanthanides were sui generis.
The problem was solved by the combined (but separate) efforts of Moseley and Bohr, the former showing that La–Lu was composed of 15 elements with atomic numbers from 57 to 71,
whilst the latter concluded that the fourth quantum shell could accommodate 32 electrons,
and that the lanthanides were associated with placing electrons into the 4f orbitals.

The Lanthanide Contraction

7

The Periodic Table places elements in atomic number order, with the lanthanides falling
between barium (56) and hafnium (72). For reasons of space, most present-day Periodic
Tables are presented with Groups IIA and IVB (2 and 4) separated only by the Group IIIB
(3) elements. Normally La (and Ac) are grouped with Sc and Y, but arguments have been
advanced for an alternative format, in which Lu (and Lr) are grouped with Sc and Y (see
e.g. W.B. Jensen, J. Chem. Educ., 1982, 59, 634) on the grounds that trends in properties
(e.g. atomic radius, I.E., melting point) in the block Sc-Y-Lu parallel those in the Group
Ti-Zr-Hf rather closely, and that there are resemblances in the structures of certain binary
compounds. Certainly on size grounds, Lu resembles Y and Sc (it is intermediate in size
between them) rather more than does La, owing to the effects of the ‘lanthanide contraction’.
The resemblances between Sc and Lu are, however, by no means complete.

1.7 The Lanthanide Contraction
The basic concept is that there is a decrease in radius of the lanthanide ion Ln3+ on crossing
the series from La to Lu. This is caused by the poor screening of the 4f electrons. This causes
neighbouring lanthanides to have similar, but not identical, properties, and is discussed in
more detail in Section 2.4.
Question 1.1 Using the information you have been given in Section 1.2, draw up a table
comparing (in three columns) the characteristic features of the s-block metals (use group 1
as typical) and the d-block transition metals.
Answer 1.1 see Table 1.3 for one such comparison.
Table 1.3 Comparison of 4f, 3d and Group I metals

Electron configurations
of ions
Stable oxidation states
Coordination numbers in
complexes
Coordination polyhedra in
complexes
Trends in coordination
numbers
Donor atoms in complexes
Hydration energy
Ligand exchange reactions
Magnetic properties of
ions
Electronic spectra of ions
Crystal field effects in
complexes
Organometallic
compounds
Organometallics in low
oxidation states
Multiply bonded atoms in
complexes

4f

3d

Group I

Variable

Variable

Noble gas

Usually +3
Commonly 8–10

Variable
Usually 6

1
Often 4–6

Minimise repulsion

Directional

Minimise repulsion

Often constant in block

Often constant in block

Increase down group

‘Hard’ preferred
High
Usually fast
Independent of
environment
Sharp lines
Weak

‘Hard’ and ‘soft’
Usually moderate
Fast and slow
Depends on environment
and ligand field
Broad lines
Strong

‘Hard’ preferred
Low
Fast
None
None
None

Usually ionic, some with
covalent character
Few

Covalently bonded

Ionically bonded

Common

None

None

Common

None

2 The Lanthanides – Principles
and Energetics
By the end of this chapter you should be able to:

r recognise the difference between f-orbitals and other types of orbitals;
r understand that they are responsible for the particular properties of the lanthanides;
r give the electron configurations of the lanthanide elements and Ln3+ ions;
r explain the reason for the lanthanide contraction;
r understand the effect of the lanthanide contraction upon properties of the lanthanides and
subsequent elements;

r explain patterns in properties such as ionization and hydration energies;
r recall that lanthanides behave similarly when there is no change in the 4f electron popr
r

ulation, but that they differ when the change involves a change in the number of 4f
electrons;
relate the stability of oxidation states to the ionization energies;
calculate enthalpy changes for the formation of the aqua ions and of the lanthanide halides
and relate these to the stability of particular compounds.

2.1 Electron Configurations of the Lanthanides and f Orbitals
The lanthanides (and actinides) are those in which the 4f (and 5f) orbitals are gradually filled.
At lanthanum, the 5d subshell is lower in energy than 4f, so lanthanum has the electron
configuration [Xe] 6s2 5d1 (Table 2.1).
As more protons are added to the nucleus, the 4f orbitals contract rapidly and become
more stable than the 5d (as the 4f orbitals penetrate the ‘xenon core’ more) (see Figure 2.1),
so that Ce has the electron configuration [Xe] 6s2 5d1 4f1 and the trend continues with Pr
having the arrangement [Xe] 6 s2 4f 3 . This pattern continues for the metals Nd–Eu, all of
which have configurations [Xe] 6s2 4f n (n = 4–7) After europium, the stability of the halffilled f subshell is such that the next electron is added to the 5d orbital, Gd being [Xe] 6s2
5d1 4f 7 ; at terbium, however, the earlier pattern is resumed, with Tb having the configuration
[Xe] 6s2 4f 9 , and succeeding elements to ytterbium being [Xe] 6s2 4f n (n = 10–14).
The last lanthanide, lutetium, where the 4f subshell is now filled, is predictably [Xe] 6s2
5d1 4f 14 .

Lanthanide and Actinide Chemistry S. Cotton


C 2006 John Wiley & Sons, Ltd.

10

The Lanthanides – Principles and Energetics

Table 2.1 Electron configurations of the lanthanides and their common ions

La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Y

Atom

Ln3+

[Xe] 5d1 6s2
[Xe] 4f1 5d1 6s2
[Xe] 4f3 6s2
[Xe] 4f4 6s2
[Xe] 4f5 6s2
[Xe] 4f6 6s2
[Xe] 4f7 6s2
[Xe] 4f7 5d1 6s2
[Xe] 4f9 6s2
[Xe] 4f10 6s2
[Xe] 4f11 6s2
[Xe] 4f12 6s2
[Xe] 4f13 6s2
[Xe] 4f14 6s2
[Xe] 4f14 5d1 6s2
[Kr] 4d1 5s2

[Xe]
[Xe] 4f1
[Xe] 4f2
[Xe] 4f3
[Xe] 4f4
[Xe] 4f5
[Xe] 4f6
[Xe] 4f7
[Xe] 4f8
[Xe] 4f9
[Xe] 4f10
[Xe] 4f11
[Xe] 4f12
[Xe] 4f13
[Xe] 4f14
[Kr]

Ln4+

Ln2+

[Xe]
[Xe] 4f1
[Xe] 4f2

[Xe] 4f4
[Xe] 4f6
[Xe] 4f7

[Xe] 4f7
[Xe] 4f8

[Xe] 4f10
[Xe] 4f13
[Xe] 4f14

Probability

4f

5d
6s

r

Figure 2.1
The radial part of the hydrogenic wave functions for the 4f, 5d and 6s orbitals of cerium (after H.G.
Friedman et al. J. Chem. Educ. 1964, 41, 357). Reproduced by permission of the American Chemical
c 1964.
Society 


2.2 What do f Orbitals Look Like?
They are generally represented in one of two ways, either as a cubic set, or as a general
set, depending upon which way the orbitals are combined. The cubic set comprises fxyz ;
fz(x2−y2) , fz(y2−z2) and fy(z2−x2); fz3 , fx3 and fy3 .
The general set, more useful in non-cubic environments, uses a different combination:
fz3 ; fxz2 and fyz2 ; fxyz ; fz(x2−y2) , fx(x2−3y2) and f y(3x2−y2) ; Figure 2.2 shows the general set.

2.3 How f Orbitals affect Properties of the Lanthanides
The 4f orbitals penetrate the xenon core appreciably. Because of this, they cannot overlap
with ligand orbitals and therefore do not participate significantly in bonding. As a result of

The Lanthanide Contraction

11
z

z

+
−

−

+

+

−

x

(a) fz 3

(b) fx z 2

y

z

−

−

+
+

+

−

−

+

+

x

x

x

−
y

(c) fx (x 2 − 3y 2)

(d) fxyz

Figure 2.2
(a) fz 3 , (fx 3 and f y 3 are similar, extending along the x- and y-axes repectively); (b) fx z 3 , (f yz 3 is similar,
produced by a 90◦ rotation about the z-axis); (c) fx(x 2 −3y 2 ) , f y(3x 2 −y 2 ) is similar, formed by a 90◦
clockwise rotation round the z-axis); (d) fx yz , (fx z 2 −y 2 ), f y(z 2 −y 2 ) and fz(x 2 −y 2 ) are produced by a 45◦
rotation about the x, y and z-axes respectively). The cubic set comprises fx 3 , f y 3 , fz 3 , fx yz , fx(z 2 −y 2 )
f y(z 2 −x 2 ) and fz(x 2 −y 2 ) ; the general set is made of fz 3 , fx z 2 , f yz 2 , fx yz , fz(x 2 −y 2 ) , fx(x 2 −3y 2 ) and f y(3x 2 −y 2 ) .
(Reproduces with permission from S.A. Cotton, Lanthanides and Actinides, Macmillan, 1991).

their isolation from the influence of the ligands, crystal-field effects are very small (and can
be regarded as a perturbation on the free-ion states) and thus electronic spectra and magnetic
properties are essentially unaffected by environment. The ability to form π bonds is also
absent, and thus there are none of the M=O or M≡N bonds found for transition metals (or,
indeed, certain early actinides). The organometallic chemistry is appreciably different from
that of transition metals, too.

2.4 The Lanthanide Contraction
As the series La–Lu is traversed, there is a decrease in both the atomic radii and in the radii of
the Ln3+ ions, more markedly at the start of the series. The 4f electrons are ‘inside’ the 5s and
5p electrons and are core-like in their behaviour, being shielded from the ligands, thus taking
no part in bonding, and having spectroscopic and magnetic properties largely independent
of environment. The 5s and 5p orbitals penetrate the 4f subshell and are not shielded from

12

The Lanthanides – Principles and Energetics
increasing nuclear charge, and hence because of the increasing effective nuclear charge
they contract as the atomic number increases. Some part (but only a small fraction) of this
effect has also been ascribed to relativistic effects. The lanthanide contraction is sometimes
spoken of as if it were unique. It is not, at least in the way that the term is usually used.
Not only does a similar phenomenon take place with the actinides (and here relativistic
effects are much more responsible) but contractions are similarly noticed on crossing the
first and second long periods (Li–Ne; Na–Ar) not to mention the d-block transition series.
However, as will be seen, because of a combination of circumstances, the lanthanides adopt
primarily the (+3) oxidation state in their compounds, and therefore demonstrate the steady
and subtle changes in properties in a way that is not observed in other blocks of elements.
The lanthanide contraction has a knock-on effect in the elements in the 5d transition
series. It would naturally be expected that the 5d elements would show a similar increase
in size over the 4d transition elements to that which the 4d elements demonstrate over the
3d metals. However, it transpires that the ‘lanthanide contraction’ cancels this out, almost
exactly, and this has pronounced effects on the chemistry, e.g. Pd resembling Pt rather than
Ni, Hf is extremely similar to Zr.

2.5 Electron Configurations of the Lanthanide Elements and of Common Ions
The electronic arrangements of the lanthanide atoms have already been mentioned (Section 2.1 and Table 2.1) where it can be seen that in general the ECs of the atoms are [Xe]
4f n 5d0 6s2 , the exceptions being La and Ce, where the 4f orbitals have not contracted
sufficiently to bring the energy of the 4f electrons below that of the 5d electrons; Gd, where
the effect of the half-filled 4f subshell dominates; and Lu, the 4f subshell having already
been filled at Yb.
In forming the ions, electrons are removed first from the 6s and 5d orbitals (rather
reminiscent of the case of the transition metals, where they are removed from 4s before
they are taken from 3d), so that all the Ln3+ ions have [Xe] 4f n arrangements.

2.6 Patterns in Ionization Energies
The values of the first four ionization energies for the lanthanides (and yttrium) are listed
in Table 2.2.
As usual, for a particular element, I4 > I3 > I2 > I1 , as the electron being removed is
being taken from an ion with an increasingly positive charge, affording greater electrostatic
attraction. Yttrium has greater ionization energies than the lanthanides as there is one fewer
filled shell, and decreased distance effects outweigh the effect of the reduced nuclear charge.
There is in general a tendency for ionization energies to increase on crossing the series
but it is irregular. The low I3 values for gadolinium and lutetium, where the one electron
removed comes from a d orbital, not an f orbital, and the high I3 value for Eu and Yb show
some correlation with the stabilizing effects of half-filled and filled f sub shells. Results
can be presented diagrammatically as cumulative ionization energies (Figure 2.3). These
diagrams demonstrate at a glance the effect of the alternately high and low values of I3 for
the elements Eu and Gd, influencing the respective stabilities of the +2 and +3 states of
these elements, similarly illustrated by the neighbours Yb and Lu. The low I4 value for Ce
(and to some extent for Pr and Tb) can be correlated with the accessibility of the Ce4+ ion.

Patterns in Ionization Energies

13

Table 2.2 Ionization Energies (kJ/mole)

La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Y

I1

I2

I3

I4

I1 + I2

I1 + I2 + I3

I1 + I2 + I3 + I4

538
527
523
529
536
543
546
593
564
572
581
589
597
603
523
616

1067
1047
1018
1035
1052
1068
1085
1167
1112
1126
1139
1151
1163
1176
1340
1181

1850
1949
2086
2130
2150
2260
2404
1990
2114
2200
2204
2194
2285
2415
2033
1980

4819
3547
3761
3899
3970
3990
4110
4250
3839
4001
4110
4115
4119
4220
4360
5963

1605
1574
1541
1564
1588
1611
1631
1760
1676
1698
1720
1740
1760
1779
1863
1797

3455
3523
3627
3694
3738
3871
4035
3750
3790
3898
3924
3934
4045
4194
3896
3777

8274
7070
7388
7593
7708
7990
8145
8000
7629
7899
8034
8049
8164
8414
8256
9740

Figure 2.3
Cumulative ionization energies across the lanthanide Series (reproduced by permission of Macmillan
from S.A. Cotton, Lanthanides and Actinides, Macmillan, 1991).

14

The Lanthanides – Principles and Energetics

2.7 Atomic and Ionic Radii
These are listed in Table 2.3 and shown in Figure 2.4. It will be seen that the atomic
radii exhibit a smooth trend across the series with the exception of the elements europium
and ytterbium. Otherwise the lanthanides have atomic radii intermediate between those
of barium in Group 2A and hafnium in Group 4A, as expected if they are represented
as Ln3+ (e− )3 . Because the screening ability of the f electrons is poor, the effective nuclear charge experienced by the outer electrons increases with increasing atomic number,
so that the atomic radius would be expected to decrease, as is observed. Eu and Yb are
exceptions to this; because of the tendency of these elements to adopt the (+2) state, they
have the structure [Ln2+ (e− )2 ] with consequently greater radii, rather similar to barium.
In contrast, the ionic radii of the Ln3+ ions exhibit a smooth decrease as the series is
crossed.
The patterns in radii exemplify a principle enunciated by D.A. Johnson: ‘The lanthanide
elements behave similarly in reactions in which the 4f electrons are conserved, and very
differently in reactions in which the number of 4f electrons change’ (J. Chem. Educ., 1980,
57, 475).
Table 2.3 Atomic and ionic radii of the lanthanides (pm)
Ba

La

Ce

Pr

Nd

Pm

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Hf

217.3 187.7 182.5 182.8 182.1 181.0 180.2 204.2 180.2 178.2 177.3 176.6 175.7 174.6 194.0 173.4 156.4
La3+ Ce3+ Pr3+ Nd3+ Pm3+ Sm3+ Eu3+ Gd3+ Tb3+ Dy3+ Ho3+ Er3+ Tm3+ Yb3+ Lu3+ Y3+
103.2 101.0 99.0
98.3 97.0 95.8 94.7 93.8 92.3 91.2 90.1 89.0 88.0 86.8 86.1 90.0

2.8 Patterns in Hydration Energies (Enthalpies) for the Lanthanide Ions
Table 2.4 shows the hydration energies (enthalpies) for all the 3+ lanthanide ions, and also
values for the stablest ions in other oxidation states. Hydration energies fall into a pattern
Ln4+ > Ln3+ > Ln2+ , which can simply be explained on the basis of electrostatic attraction,
210

Radius (pm)

190
170
Ln3+ radius/pm
metalic radius/pm

150
130
110
90

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Figure 2.4
Metallic and ionic radii across the lanthanide series.

Enthalpy Changes for the Formation of Simple Lanthanide Compounds

15

since the ions with a larger charge have a greater charge density. The hydration energies
for the Ln3+ ions show a pattern of smooth increase with increasing atomic number, as the
ions becomes smaller and their attraction for water molecules increases. This is another
example of the principle enunciated by Johnson.

2.9 Enthalpy Changes for the Formation of Simple Lanthanide Compounds
Observed patterns of chemical behaviour can sometimes appear confusing or sometimes
just be encapsulated in rules [e.g. Ce has a stable (+4) oxidation state]. They can frequently
be explained in terms of the energy changes involved in the processes.

2.9.1 Stability of Tetrahalides
Among the fluorides, lanthanum forms only LaF3 , whilst cerium forms CeF3 and CeF4 .
(Tetrafluorides are also known for Pr and Tb, see Section 3.4). Why do neighbouring metals
behave so differently? First, examine the energetics of formation of LaF3 using a Born–
Haber cycle.
H (kJ/mol)
La(s) → La(g)
+402
3+
La(g) → La (g)
+538 + 1067 + 1850
3/2F2 (g) → 3F (g)
+252
3F(g) + 3 e → 3F− (g)
−984
La3+ (g) + 3F− (g) → LaF3
−4857
Thus La(s) + 3/2F2 (g) → LaF3
H = +402 + (538 + 1067 + 1850) + 252 − 984 − 4857 = −1732 kJ/mol
Calculated and observed enthalpies of formation for LnXn (n = 2–4) are given in Table 2.5.
The same method can be used to calculate Hf for LaF4 , making the assumption that
the lattice energy is the same as that for CeF4 (−8391 kJ/mol), and that I4 for lanthanum is
+4819 kJ/mol. In this case, Hf (LaF4 ) = −691 kJ/mol. Similarly, using the same method,
Hf for LaF2 can be calculated as H = −831 kJ/mol (assuming that the lattice energy
is the same as for BaF2 (−2350 kJ/mol). This poses the question: if Hf for LaF2 and for
LaF4 are −831 and −691 kJ/mol respectively, why can’t these compounds be isolated?
Apart from the oversimplification of using H rather than G values as an index of
stability, what the preceding calculations have done is to indicate that these compounds
are stable with respect to the elements, and no other decomposition pathways have been
considered.
Table 2.4 Enthalpies of hydration of the lanthanide ions (values given as −H hydr/kj mol−1
La3+

Ce3+

Pr3+

Nd3+

Pm3+

Sm3+

Eu3+

Gd3+

3278 3326 3373 3403
Ce4+
6309

3427

3449
Sm2+
1444

3501 3517
Eu2+
1458

Tb3+

Dy3+

Ho3+

Er3+

Tm3+

Yb3+

Lu3+

Y3+

3559 3567

3623

3637 3664

3706
Yb2+
1594

3722 3583

16

The Lanthanides – Principles and Energetics

Table 2.5 Enthalpies of formation of lanthanide halidesa

La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu

LnF2

LnF3

LnF4

LnCI2

LnCI3

LnCI4

LnBr2

LnBr3

LnI2

LnI3

880
950
1050
1050
1080
1160
1188
870
980
1050
1040
1020
1090
1172
790

1732
1733
1712
1661
1700
1669
1584
1699
1707
1678
1698
1699
1689
1570
1640

600
1946
1690
1500
1500
1450
1290
1310
1742
1540
1500
1510
1500
1250
1210

520
580
700
707
720
820
824
500
600
693
660
640
709
799
420

1073
1058
1059
1042
1040
1040
1062
937
1008
1007
990
995
995
960
986

−480
820
630
490
430
400
230
180
600
420
350
360
360
250
160

430
490
610
630
620
720
760
400
500
590
580
560
640
710
330

907
890
891
873
850
857
799
829
850
831
830
836
840
800
850

320
380
490
510
510
600
630
290
390
470
450
420
500
580
190

699
686
678
665
640
640
540
619
598
603
594
586
582
561
556

are quoted as −Hf (kJ/mol).
Experimental values in bold.
Values taken from D.W. Smith, J. Chem. Educ., 1986, 63, 228.

a Values

LaF4 might decompose thus :
LaF4 → LaF3 + 1/2 F2
Applying Hess’s Law to this in the form
Hreaction = gH f (products) − H f (reactants)
Hreaction = −1732 − (−691 + 0) = −1041 kJ/mol
This decomposition is thus thermodynamically favourable (especially as it would be
favoured on entropy grounds too, with the formation of fluorine gas)
LaF2 might decompose by disproportionation:
3LaF2 → 2LaF3 + La
Using Hf for LaF2 and for LaF3 (−831 and −1732 kJ/mol respectively), H for this
decomposition reaction can be calculated as −971 kJ/mol. This is again a very exothermic
process, indicating that LaF2 is likely to be unstable (this is analogous to the reason for
the non-existence of MgCl, as disproportionation into Mg and MgCl2 is favoured, as the
reader may be aware). The reason for this is that, although I3 for lanthanum is large (and
endothermic), it is more than compensated for by the higher lattice energy for LaF3 compared
with the value for LaF2 .
Since both CeF3 and CeF4 are isolable, what makes the difference here? If similar calculations are carried out (assuming lattice energies for CeF3 and CeF4 of −4915 and −8391
kJ/mol respectively, and an enthalpy of atomization of 398 kJ/mol for Ce (see Table 2.6),
and using the same enthalpy of atomization and electron affinity for fluorine as in the lanthanum examples), Hf for CeF3 can be calculated as −1726 kJ/mol and Hf for CeF4 as
−1899 kJ/mol.
The discrepancy between Hf for CeF3 and CeF4 is much smaller than is the case for
lanthanum. In fact, for the decomposition reaction
CeF4 → geF3 + 1/2 F2

Enthalpy Changes for the Formation of Simple Lanthanide Compounds

17

Table 2.6 Enthalpies of atomization of the lanthanides (kJ/mol)
Ba
150.9

La

Ce

Pr

Nd

402.1

398

357

328

Pm

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Hf

164.8

176

301

391

293

303

280

247

159

428.0

570.7

H = −1726 − (−1899) = +173 kJ/mol, making the decomposition of CeF4 relatively
unfavourable. CeF4 is stable, certainly at ambient temperatures.
If the values of the parameters used in the calculation are compared, the determining
factor is the much lower value of I4 for cerium (3547 kJ/mol compared with 4819 kJ/mol
for La) due to the fact that the fourth electron is being removed from a different shell (nearer
the nucleus) in the case of lanthanum.
Other tetrahalides do not exist. Thus, though both CeCl3 and salts of the [CeCl6 ]2− ion
can be isolated, CeCl4 cannot be made. The reasons for this are those that enable fluorine to
support high oxidation states (see Question 2.4). Similar factors indicate that tetrabromides
and tetraiodides are much less likely to be isolated.

2.9.2 Stability of Dihalides
The stability of the dihalides can be explained in a similar way. As already noted, LaF2 does
not exist. When might dihalides be expected?
One way in which dihalides can decompose is disproportionation
3 LnX2 → 2 LnX3 + Ln
though a number of dihalides (particularly dichlorides) have been made by the reverse
reproportionation, by heating the mixture to a high temperature and rapidly quenching
(Section 3.5.1.)
2 LnX3 + Ln → 3 LnX2
It can be shown, using Hess’s Law, that the disproportionation will be exothermic unless:
Hf LnX3 /Hf LnX2 < 1.5
The disproportionation can be broken down into individual components (Figure 2.5)
H1 = H2 + H3 + H4
H1 = [−3Hlatt (LnX2 )] + {2 I3 (Ln) − [I1 (Ln) + I2 (Ln)]}
+ [−Hat (Ln) + 2 Hlatt (LnX3 )]
H1 = {2 I3 (Ln) − [I1 (Ln) + I2 (Ln)]} + 2 Hlatt (LnX3 ) − 3 Hlatt (LnX2 ) − Hat (Ln)
3 LnX2(s)

∆H1

2 LnX3(s) + Ln(s)

∆H2

2+

∆H4

−

3 Ln (g) + 6X (g)

Figure 2.5
Disproportion of lanthanide dihalides.

∆H3

3+

−

2 Ln (g) + 6X (g) + Ln(g)

18

The Lanthanides – Principles and Energetics
These equations can be used qualitatively, first to suggest for which lanthanides the halides
LnX2 are most likely to be stable. For the disproportionation process to be more likely to be
endothermic (i.e. stabilizing the +2 state), the preceding equation suggests that high values
of I3 are favourable. Study of Table 2.2 shows that this is more likely to be associated with
Eu, Sm and Yb. Iodide is the halide most often found in low oxidation state halides. In the
case of LnI2 , the large size of the iodide ion will reduce the lattice energy for both LnI2 and
LnI3 so that the difference in lattice enthalpy will become less significant, favouring LnI2 .
The dihalide will also be favoured by a low Hat (Ln), again associated with the lanthanides
most often found in the +2 state, Eu and Yb (see Table 2.6).
Applying the Hf LnX3 /Hf LnX2 <1.5 criterion, and using data in Table 2.5, the
halides LnX2 are most likely to be stable are predicted to be LnF2 (Ln = Sm, Eu, Yb);
LnCl2 (Ln = Nd, Pm, Sm, Eu, Dy, Tm, Yb); LnY2 (Y = Br, I; Ln = Pr, Nd, Pm, Sm, Eu,
Dy, Ho, Er, Tm, Yb).
The known dihalides are listed in Table 3.2. There is a reasonably good correlation, given
that the Pm dihalides have not been investigated on account of promethium’s short half-life.
Some dihalides listed are ‘metallic’ and are not covered by this argument.

2.9.3 Stability of Aqua Ions
Since Ln3+ (aq) is the most stable aqua ion, then both of the following processes are favoured.
Ln2+ (aq) + H+ (aq) → Ln3+ (aq) + 1/2 H2 (g)
and 2 Ln4+ (aq) + H2 O(aq) → 2 Ln3+ (aq) + 2 H+ (aq) + 1/2 O2 (g)
In other words, Ln2+ ions tend to reduce water and Ln4+ ions tend to oxidize it. We can
examine the stability of the Ln2+ ion using a treatment similar to the one just employed
(Figure 2.6), with



Hox (Ln2+ ) = I3 + [Hhydr [Ln3+ (aq)] − Hhydr [Ln2+ (aq)] − 439 kJ/mol.
When is it most likely that Ln2+ (aq) ions will be stable? For the first of the two reactions above to be favoured, the single factor that will help make H positive is a
high value of I3 . Less important would be the size of the ions, as this could affect the
hydration enthalpies; the difference between the hydration enthalpies will be less, the
larger the lanthanide ions. Substituting into the above equation, we can investigate the
relative stabilities of La2+ (aq) and Eu2+ (aq), making use of ionization energies from
∆ Hhydr (Ln2+ )
Ln2+(g)

+

Ln2+(aq) + H (aq)

I3

Ln3+(g)

Figure 2.6
Oxidation of Ln2+ (aq).

∆HOX(Ln2+) + ∆HH

∆ Hhydr (Ln3+)

Ln3+(aq) + 1/2H2(g)

Patterns in Redox Potentials

19

Table 2.2 and enthalpies of hydration found in Table 2.4, also assuming Hhydr [(La
2+
(aq)] = −1327 kJ/mol:
For La2+ :
Hox (La2+ ) = I3 + {Hhydr [La3+ (aq)] − Hhydr [La2+ (aq)]} − 439
= 1850 + [−3278 − (−1327)] − 439
= −540 kJ/mol
For Eu2+ :
Hox (Eu2+ ) = I3 + {Hhydr [Eu3+ (aq)] − Hhydr [Eu2+ (aq)]} − 439
= 2404 + [−3501 − (−1458)] − 439
= −78 kJ/mol
The large exothermic value for Hox (La2+ ) indicates that it is not likely to exist in aqueous
solution. The Eu2+ (aq) ion is known to have a short lifetime in water, even though H
is negative for the oxidation process, so the activation energy for oxidation may be rather
high.

2.10 Patterns in Redox Potentials
Known and estimated values are listed in Table 2.7. The values for the reduction potential
for Ln3+ + 3 e− → Ln are very consistent, with slight irregularities at Eu and Yb. The
potential largely depends upon three processes:
Ln(s) → Ln(g)

Hat (Ln)

Ln(g) → Ln (g) + 3 e−
3+

Ln (g) → Ln (aq)
3+

3+

I 1 + I 2 + I3

Hhydr (Ln3+ )

The first two of these are endothermic and the third exothermic; overall H is the difference
between two large quantities. It remains fairly constant across the series, apart from Eu and
Yb, with values of 608 (La); 712 (Eu); 630 (Gd); 613 (Tm); 644 (Yb) and 593 (Lu) kJ/mol
being representative, the values for Eu and Yb resulting from the high I3 values. The very
negative value for the reduction potential is expected for such reactive metals (and also
reflects the difficulty in isolating them). The potentials for the Ln3+ + e− → Ln2+ process
reflect the stability of the +2 state. Since I3 relates to H for the process, and G = −nFE,
a relationship between these is unsurprising. Similarly, the only potential for the Ln4+ +
e− → Ln3+ process within reasonable range is that for Ce, and indicates that Ce4+ is the
only ion in this state likely to be encountered in aqueous solution.
Question 2.1 What is the trend in atomic radii, and that in ionic radii, for the lanthanides?
What are the exceptions to this, and why?
Answer 2.1 The structure of metals is usually described as one in which metal ions are
surrounded by a ‘sea’ of delocalised outer-shell electrons. The greater the number of loosely
held electrons, the stronger the metallic bonding and the smaller the atomic radius. If the

Ce

Pr

b=

in parentheses are estimated.
in THF.

a Values

Ln3+ + 3e → Ln −2.37 −2.34 −2.35
Ln3+ + e → Ln2+ (−3.1) (−3.2) (−2.7)
1.70 (3.4)
Ln4+ + e → Ln3+

La

Pm

−2.32 −2.29
−2.6b (−2.6)
(4.6)
(4.9)

Nd

Table 2.7 Redox potentials of the lanthanide ions (V)a
−2.30
−1.55
(5.2)

Sm

Gd

Tb

−1.99 −2.29 −2.30
−0.34 (−3.9) (−3.7)
(6.4)
(7.9)
(3.3)

Eu

Ho

Er

−2.29 −2.33 −2.31
−2.5b (−2.9) (−3.1)
(5.0)
(6.2)
(6.1)

Dy

−2.31
−2.3b
(6.1)

Tm

−2.22
−1.05
(7.1)

Yb

Y

(8.5)

−2.30 −2.37

Lu

Patterns in Redox Potentials

21

lanthanides are represented as Ln3+ (e− )3 , then the atomic radii would be expected to fall
between those of barium [Ba2+ (e− )2 ] and hafnium [Hf 4+ (e− )4 ], as is generally observed.
Question 2.2 Using Hess’s Law and the Hf for LaF2 (−831 kJ/mol) and for LaF3 (−1732
kJ/mol), calculate H for this decomposition reaction.
3 LaF2 → 2 LaF3 + La
Answer 2.2
Hreaction = Hf (products) − Hf (reactants)
Hreaction = 2 (−1732) − 3 (−831 + 0) = −971 kJ/mol
Question 2.3 Assuming lattice energies for CeF3 and CeF4 of −4915 and −8391 kJ/mol
respectively, and using the same enthalpy of atomization and electron affinity for fluorine
as in the lanthanum examples (Section 2.9.1.), calculate Hf for CeF3 and CeF4 . Take an
enthalpy of atomization of 398 kJ/mol for cerium (Table 2.6). Use ionization energies from
Table 2.2.
Answer 2.3
H (kJ/mol)
+398
+527 + 1047 + 1949
+252
−984
−4915

Ce(s) → Ce (g)
Ce(g) → Ce3+ (g)
3/2 F2 (g) → 3F(g)
3F(g) + 3e → 3F− (g)
Ce3+ (g) + 3F− (g) → CeF3 (s)
Thus Ce(s) + 3/2 F2 (g) → CeF3 (s)
H = +398 + (527 + 1047 + 1949) + 252 − 984 − 4915 = −1726 kJ/mol

H (kJ/mol)
+398
+527 + 1047 + 1949 + 3547
+336
−1312
−8391

Ce(s) → Ce(g)
Ce(g) → Ce4+ (g)
2F2 (g) → 4F (g)
4F(g) + 4e → 4F− (g)
Ce4+ (g) + 4F− (g) → CeF4 (s)
Thus Ce(s) + 2F2 (g) → CeF4 (s)
H = +398 + (527 + 1047 + 1949 + 3547) + 336 − 1312 − 8391 = −1899 kJ/mol

Question 2.4 Unlike CeF4 , CeCl4 does not exist, though CeCl3 does. Suggest why this
might be. Values of Hf for CeCl3 and CeCl4 are −1058 and −820 (calculated) kJ/mol,
respectively.
Answer 2.4 Fluorine is well known to promote high oxidation states. Factors associated
with this are the high lattice energies associated with the small fluoride ion, along with the
very small F–F bond energy (due to non-bonding electron pair repulsions) as well as high
bond energies involving fluorine (not relevant in this case). Because of the larger size of the
Cl− ion, there is going to be much less difference between the lattice energies of CeCl3 and

22

The Lanthanides – Principles and Energetics
=E
= I3

0

1800

3

2000

2

2200

1

2400

E 0(V)

4

I3(kJ mol−1)

Key:

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Figure 2.7
Strong correlation between I3 and E ◦ for Ln3+ + e → Ln2+ (reproduced by permission of Macmillan
from S.A. Cotton, Lanthanides and Actinides, 1991, p. 26.)

CeCl4 , and this is probably the determining factor. The higher Hat (Cl) of 121.5 kJ/mol
also mitigates against the formation of CeCl4 .
Question 2.5 Applying the Hf LnX3 /Hf LnX2 < 1.5 criterion for stability of dihalides,
and using data in Table 2.5, predict which the halides LnX2 are most likely to be exist.
Answer 2.5 As in section 2.9.2 LnF2 (Ln = Sm, Eu, Yb); LnCl2 (Ln = Nd, Pm, Sm, Eu,
Dy, Tm, Yb); LnY2 (Y = Br, I; Ln = Pr, Nd, Pm, Sm, Eu, Dy, Ho, Er, Tm, Yb) are predicted
to be stable, in reasonably good agreement with the currently known facts.
Exercise 2.6 Using the same horizontal axis (atomic numbers 57–71), plot values of (a) the
third ionization enthalpy I3 and (b) the Ln3+ + e → Ln2+ reduction potential on the y axis
(choose appropriate scales). Comment.
Answer 2.6 There is a strong correlation between them, not surprisingly! See Figure 2.7.

3 The Lanthanide Elements and
Simple Binary Compounds
By the end of this chapter you should be able to:

r know how to prepare lanthanide metals and simple binary compounds such as the halides,
oxides, and hydrides;
the principle of the lanthanide contraction to explain patterns in co-ordination
number in these compounds;
apply knowledge gained in the study of Chapter 2 to the compounds in unusual oxidation
states;
understand the uses of the metals and certain compounds in applications such as hydrogen
storage and in superconductors.

r apply
r
r

3.1 Introduction
This chapter discusses the synthesis of the lanthanide metals, their properties, reactions, and
uses. It also examines some of the most important binary compounds of the lanthanides,
particularly the halides, which well illustrate patterns and trends in the lanthanide series.

3.2 The Elements
3.2.1 Properties
The lanthanides are rather soft reactive silvery solids with a metallic appearance, which tend
to tarnish on exposure to air. They react slowly with cold water and rapidly in dilute acid.
They ignite in oxygen at around 150–200 ◦ C; similar reactions occur with the halogens,
whilst they react on heating with many nonmetals such as hydrogen, sulfur, carbon, and
nitrogen (above 1000 ◦ C).
The metals are relatively high-melting and -boiling. Their physical properties usually
show smooth transition across the series, except that discontinuities are often observed for
the metals that have a stable +2 state, europium and ytterbium. Thus the atomic radii of
europium and ytterbium are about 0.2 Å greater than might be predicted by interpolation
from values for the flanking lanthanides (Figure 3.1).
Similarly, Sm, Eu, and Yb have boiling points that are lower than those of the neighbouring
metals (Figure 3.2).

Lanthanide and Actinide Chemistry S. Cotton

C 2006 John Wiley & Sons, Ltd.

24

The Lanthanide Elements and Simple Binary Compounds

Figure 3.1
The atomic radii of the lanthanide metals (reproduced with permission from S.A. Cotton, Lanthanides
and Actinides, Macmillan, 1991).
4000

Bp (K)

3000

2000

1000
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Figure 3.2
The boiling points of the lanthanide metals (reproduced with permission from S.A. Cotton,
Lanthanides and Actinides, Macmillan, 1991).

Assuming that one can represent the structure of a metal as a lattice of metal ions permeated by a sea of electrons, then metals like lanthanum can be shown as (Ln3+ ) (e− )3 ;
however, metals based upon divalent ions (like Eu and Yb) would be (Ln2+ ) (e− )2 . The ions
with the (+3) charge have a smaller radius, as the higher charge draws in the electrons more
closely, and the stronger attraction means that it takes more energy to boil them (similarly,
they would be predicted to have higher conductivities).

3.2.2 Synthesis
The metals have similar reactivities to magnesium. This means that they cannot be extracted
by methods like carbon reduction of the oxides; in practice, metallothermic reduction of
the lanthanide fluoride or chloride with calcium at around 1450 ◦ C is used. The product

Binary Compounds

25

is an alloy of (excess of) calcium and the lanthanide, from which the calcium can be
distilled.
2 LnF3 + 3 Ca → 2 Ln + 3 CaF2
The reduction is carried out under an atmosphere of argon, not nitrogen.
The above method is not suitable for obtaining the metals with a stable +2 state, which
are only reduced as far as the difluoride (Eu, Yb, Sm). The lanthanide can be removed by
distillation.
2 La + M2 O3 
 2 M + La2 O3
The ‘divalent’ metals Sm, Eu, and Yb have boiling points of 1791, 1597 and 1193 ◦ C
respectively, much lower than that of La (3457 ◦ C), so that on heating they are distilled off,
their volatility meaning that their removal from the mixture will displace the equilibrium
to the right, so the reaction will proceed to completion.

3.2.3 Alloys and Uses of the Metals
Mischmetal is the lanthanide alloy with the longest history. It is a mixture of the lighter
lanthanides, cerium in particular, which is manufactured from an ‘unseparated’ mixture
of the oxides. This is first converted into a mixture of the anhydrous chlorides, which is
then electrolysed using a graphite anode and iron cathode at around 820 ◦ C to afford the
mixture of metals. This is used mainly as an alloy with iron for the desulfurization and
deoxidation of steels and more familiarly in cigarette lighter flints. SmCo5 and other alloys
of these metals have been used to make extremely strong permanent magnets. LaNi5 has
been widely examined as a material for hydrogen storage, with applications in fuel cells,
catalytic hydrogenation, and removal of hydrogen from gas mixtures, as it rapidly absorbs
hydrogen at room temperature to afford compositions up to LaNi5 H6 ; the hydrogen is given
up quickly at 140 ◦ C. The most important application lies in rechargeable batteries for PCs
(see Section 3.10).

3.3 Binary Compounds
3.3.1 Trihalides
Most halides are LnX3 , but a number of LnX2 are known, as are a handful of tetrafluorides.

Syntheses of the Trihalides
Although the halides can be obtained as hydrates from reaction of the metal oxides or
carbonates with aqueous acids, these hydrates are hydrolysed on heating to the oxyhalide
and thus the anhydrous halides (which are themselves deliquescent) cannot be made that
way.
LnX3 + H2 O → LnOX + HX
The fluorides are obtained as insoluble hydrates LnF3 .0.5H2 O by precipitation and the
hydrates dehydrated by heating in a current of anhydrous HF gas (or in vacuo).
LnF3 .0.5H2 O → LnF3 + 0.5 H2 O

26

The Lanthanide Elements and Simple Binary Compounds
Otherwise the anhydrous halides can generally be made by heating the metal with the
halogen (except for EuI3 ) or gaseous HCl.
2 Ln + 3 X2 = 2 LnX3
Another method for the chlorides involves refluxing the hydrated chlorides with thionyl
dichloride (SOCl2 ) for a few hours; an advantage of this method is that the other reaction
products are gaseous SO2 and HCl.
LnCl3 .xH2 O + x SOCl2 → LnCl3 + x SO2 + 2x HCl
A route that works well in practice involves thermal decomposition of ammonium
halogenometallates. Adding ammonium chloride to a solution of the metal oxides in hydrochloric acid, followed by evaporation gives halogenometallate salts. These can be dehydrated by heating with excess of ammonium chloride in a stream of gaseous HCl, the
resulting anhydrous salt being decomposed by heating in vacuo at about 300 ◦ C.
Ln2 O3 + 9 NH4 Cl + 3 HCl → 2 (NH4 )3 LnCl6 + 3 NH3 + 3 H2 O
2 (NH4 )3 LnCl6 → 2 LnCl3 + 6 NH4 Cl
The anhydrous halides can be purified by sublimation in vacuo, but owing to a tendency to
react with silica, contact with hot glass, thereby forming the oxyhalide, should be avoided.

Structures of the Trihalides
The lanthanide trihalides demonstrate very clearly the effect of varying the cation and anion
radii upon the structure type adopted (Table 3.1).
The early lanthanide fluorides adopt the ‘LaF3 ’ structure (Figure 3.3) based on a metal
ion surrounded by a trigonal prism of fluorides with two additional capping fluorides, giving
11 coordination (9 + 2), whilst the later fluorides have the ‘YF3 ’ structure. This is based
on tricapped trigonal prismatic 9 coordination, in which the prism is somewhat distorted.
The structure adopted by UCl3 and several of the lanthanide halides is again a tricapped
Table 3.1 Structures of the lanthanide trihalides

La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Y

F

Cl

Br

I

LaF3 (11)
LaF3 (11)
LaF3 (11)
LaF3 (11)
LaF3 (11)
YF3 (9)
YF3 (9)
YF3 (9)
YF3 (9)
YF3 (9)
YF3 (9)
YF3 (9)
YF3 (9)
YF3 (9)
YF3 (9)
YF3 (9)

UCl3 (9)
UCl3 (9)
UCl3 (9)
UCl3 (9)
UCl3 (9)
UCl3 (9)
UCl3 (9)
UCl3 (9)
PuBr3 (8)
AlCl3 (6)
AlCl3 (6)
AlCl3 (6)
AlCl3 (6)
AlCl3 (6)
AlCl3 (6)
AlCl3 (6)

UCl3 (9)
UCl3 (9)
UCl3 (9)
PuBr3 (8)
PuBr3 (8)
PuBr3 (8)
PuBr3 (8)
FeCl3 (6)
FeCl3 (6)
FeCl3 (6)
FeCl3 (6)
FeCl3 (6)
FeCl3 (6)
FeCl3 (6)
FeCl3 (6)
FeCl3 (6)

PuBr3 (8)
PuBr3 (8)
PuBr3 (8)
PuBr3 (8)
PuBr3 (8)
FeCl3 (6)

Values in parentheses indicate the coordination number.

FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3

(6)
(6)
(6)
(6)
(6)
(6)
(6)
(6)
(6)

Binary Compounds

27

Figure 3.3
The LaF3 structure (reproduced by permission of Macmillan from S.A. Cotton, Lanthanides and
Actinides, 1991, p. 42).

Figure 3.4
The structure of UCl3 and certain LnCl3 [reproduced in modified form, from R.B. King (ed.),
Encyclopedia of Inorganic Chemistry, 1st edition, Wiley, Chichester; 1994].

trigonal prism (Figure 3.4); if one of the face-capping halogens is removed from the ‘UCl3 ’
structure, the 8 coordinate PuBr3 structure is generated. Finally, the AlCl3 and BiI3 (also
sometimes referred to as ‘FeCl3 ’) structures both have octahedral six-coordination.
The trends are similar to those found in halides of the Group I and Group II metals
and are explained on an ionic packing model; more anions can be packed round the large
lanthanide ions early in the series than around the smaller, later ones; similarly, more of
the small fluoride ions can be packed round a given lanthanide ion than is the case with the
much larger iodide anion.

Properties of the Trihalides
The trihalides are high-melting solids, with many uses in synthetic chemistry, though their
insolubility in some organic solvents means that complexes, such as those with thf (Section
4.3.3), are often preferred.

3.3.2 Tetrahalides
Only the fluorides of Ce, Pr, and Tb exist, the three lanthanides with the most stable (+4)
oxidation state. Fluorine is most likely to support a high oxidation state, and even though
salts of ions like [CeCl6 ]2− are known, the binary chloride has not been made. CeF4 can be
crystallized from aqueous solution as a monohydrate. Anhydrous LnF4 (Ln = Ce, Pr, Tb)

28

The Lanthanide Elements and Simple Binary Compounds
can be made by fluorination of the trifluoride or, in the case of Ce, by fluorination of
metallic Ce or CeCl3 . All three tetrafluorides have the MF4 structure with dodecahedral
eight coordination. Factors that favour formation of a tetrafluoride include a low value of
I4 for the metal and a high lattice energy (see Section 2.9.1). This is most likely to be found
with the smallest halide ion, fluoride. The low bond energy of F2 is also a supporting factor.

3.3.3 Dihalides
These are most common for metals with a stable (+2) state, such as Eu, Yb, and Sm.
As would be expected, they most often occur for the iodide ion, the best reducing agent.
A number of dihalides are known for other metals, though some of these do not actually
involve the (+2) oxidation state.

Synthetic Routes
These compounds are usually made by reduction using hydrogen (e.g. EuX2 , YbX2 , or
SmI2 ) or reproportionation.
2 EuCl3 + H2 → 2 EuCl2 + 2 HCl
2 DyCl3 + Dy → 3 DyCl2
In a few cases, thermal decomposition is applicable [LnI2 (Ln = Sm, Yb); EuBr2 ].
2 YbI3 → 2 YbI2 + I2
Another method, used especially for diiodides, involves heating the metal with HgX2 .
Tm + HgI2 → TmI2 + Hg
The known dihalides are listed in Table 3.2, together with an indication of the structure and
coordination number.
The iodides of Nd, Sm, Eu, Dy, Tm, and Yb are definitely compounds of the +2 ions, with
salt-like properties, are insulators, and have magnetic and spectroscopic properties expected
Table 3.2 Structures of the lanthanide dihalides
F
La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
a

Cl

Br

I
a

MoSi2 (8)
MoSi2 (8)
a
MoSi2 (8)
SrBr2 (7,8)
a

CaF2 (8)
CaF2 (8)

CaF2 (8)

PbCl2 (9)

PbCl2 (9)

PbCl2 (9)
PbCl2 (9)

PbCl2 (9); SrBr2 (7,8)
SrBr2 (7,8)

EuI2 (7)
EuI2 (7)
a
MoSi2 (8)

SrBr2 (7,8)

SrI2 (7)

CdCl2 (6)

SrI2 (7)
SrI2 (7)

SrI2 (7)
SrI2 (7); CaCl2 (6)

CdI2 (6)
CdI2 (6)

LnIII compounds with the structure M3+ (I− )2 (e− ).

Binary Compounds

29

for the M2+ ions (thus electronic spectra of EuX2 resemble those of the isoelectronic Gd3+
ions). SmI2 is proving an important reagent in synthetic organic chemistry (Section 8.3).
Several of the diiodides, however, have a metallic sheen and are very good conductors of
electricity, such as LaI2 , CeI2 , PrI2 , and GdI2 . Since they are good conductors in the solid
state, the presence of delocalized electrons is indicated. M3+ (I− )2 (e− ) is a likely structure.
None of these metals exhibits a stable +2 state in any of its compounds and consideration of
factors such as their ionization energies suggest they are unlikely to form stable compounds
in this state (see Section 2.9.2).

3.3.4 Oxides
Oxides M2 O3
As already mentioned, the metals burn easily, rather like Group 2 metals, forming oxides
(but with some nitride). The oxides are therefore best made by thermal decomposition of
compounds like the nitrate or carbonate.
4 Ln(NO3 )3

→

2 Ln2 O3 + 12 NO2 + 3 O2

Most lanthanides form Ln2 O3 , but those metals with accessible +4 and +2 oxidation states
can afford other stoichiometries. These may be turned into Ln2 O3 by synthesis under a
reducing atmosphere. CeO2 can be reduced to Ce2 O3 using hydrogen. The oxides are somewhat basic and absorb CO2 from the atmosphere, forming carbonates, and water vapour,
forming hydroxides. As expected they dissolve in acid, forming salts, and are convenient
starting materials for the synthesis of lanthanide salts, including the hydrated halides.
The sequioxides Ln2 O3 adopt three structures, depending upon the temperature and
upon the lanthanide involved. At room temperature, La2 O3 to Sm2 O3 inclusive adopt the
A-type structure, which has capped octahedral 7 coordination of the lanthanide. The B-type
structure tends to be exhibited by La2 O3 to Sm2 O3 at higher temperatures and has three
different lanthanide sites, one with distorted 6 coordination and the others with face-capped
octahedral 7 coordination. The C-type structure is followed by heavier metals and has 6
coordination, severely distorted from an octahedron.

Oxides MO2
Cerium, having the stablest (+4) state, is the only metal to form a stoichiometric oxide in
this state, CeO2 , which has the fluorite structure. It is white when pure, but even slightly
impure specimens tend to be yellow. It can be made by burning cerium or heating salts
like cerium(III) nitrate strongly in air. It is basic and dissolves with some difficulty in acid,
forming Ce4+ (aq), which can be isolated as salts such as the nitrate Ce(NO3 )4 .5H2 O. Uses
of CeO2 include self-cleaning ovens and as an oxidation catalyst in catalytic converters.
Praseodymium and terbium form higher oxides, of which a number of phases are known
between Ln2 O3 and LnO2 . Ignition of praseodymium nitrate leads to Pr2 O3 but further
heating in an oxygen atmosphere gives Pr6 O11 or even PrO2 ; terbium similarly yields
Tb4 O7 and TbO2 .

Oxides MO
Reduction of Eu2 O3 with Eu above 800 ◦ C (comproportionation) gives EuO.
Eu2 O3 + Eu → 3 EuO

30

The Lanthanide Elements and Simple Binary Compounds
This compound and the similar YbO have salt-like (NaCl) structures and are genuine LnII
compounds, being insulators. Similar comproportionation methods using high pressures
have been used to obtain SmO and NdO; these are shiny conducting solids, probably
containing Ln3+ ions.

Oxide Superconductors
Superconductivity is the phenomenon in which a material conducts electricity with virtually zero resistance. For many years until the mid 1980s, the highest temperatures available
were around 20 K and required expensive liquid helium (or hydrogen) coolant. If hightemperature superconductors can be made, this has obvious application in area such as
power transmission. In 1986, Bednorz and Muller reported that La1.8 Sr0.2 CuO4 was a superconductor up to 38 K. This sparked intense world-wide activity, and the following year
Wu, Chu and others reported that YBa2 Cu3 O7−δ (0 ≤ δ ≤ 1) had a Tc (superconducting
transition temperature) of 92 K, bringing the phenomenon into the liquid nitrogen range.
Steady though less spectacular advances have produced materials like HgBa2 Ca2 Cu3 O8
(Tc = 133K).
The structure of YBa2 Cu3 O7 is based on an oxygen-deficient layered perovskite structure
and features two types of copper environment (Figure. 3.5), formally containing Cu2+ and
Cu3+ , with both square planar and square pyramidal coordination. Removing the shaded
oxygens results in phases down to the semiconductor YBa2 Cu3 O6 . The mechanism of
superconductivity is still debated, but one theory suggests that it involves an electron passing
through the lattice distorting it in such a way that a second electron follows closely in its wake

Figure 3.5
The structure of YBa2 Cu3 O7 ; the shaded atoms are those removed to create oxygen-deficient phases
up to YBa2 Cu3 O6 (reproduced by permission of Macmillan from S.A. Cotton, Lanthanides and
Actinides, 1991, p. 47).

Hydrides

31
with no hindrance to its passing, the electron pair being known as a ‘Cooper pair’. Despite
intense activity over the last 15 years no truly commercial material has yet emerged, due to
the intrinsically brittle nature of the oxide ceramic materials making fabrication into wires
impossible. Current thinking favours making thin films, though with present technology
they will only have low current-carrying capacity.

3.4 Borides
A number of stoichiometries obtain, such as LnB2 , LnB4 , LnB6 , LnB12 , and LnB66 .
The most important are LnB6 . Borides are obtained by heating the elements together
at 2000 ◦ C or by heating the lanthanide oxide with boron or born carbide at 1800 ◦ C.
They are extremely unreactive towards acids, alkalis, and other chemicals, and are metallic
conductors. They contain a continuous three-dimensional framework of (B6 )2− octahedra
interspersed with Ln3+ ions, indicating an electronic structure (Ln3+ ) (B6 )2− (e− ) in most
cases, though EuB6 and YbB6 are insulators, suggesting them to be (Ln2+ ) (B6 )2− . Because
of its high thermal stability and melting point (2400 ◦ C), and its metal-like electrical and
thermal conductivity, LaB6 is an important thermionic emitter material for the cathodes of
electron guns. Mixed boride materials are also of commercial importance. Nd–Fe–B alloys,
with compositions such as Nd2 Fe14 B, are the strongest permanent magnet materials available today. Other important compounds include lanthanide rhodium boride low-temperature
superconductors (such as ErRh4 B4 ), part of a wider LnM4 B4 family (M = transition metal).

3.5 Carbides
The lanthanides form carbides with a range of compositions, notably LnC2 , but additionally
Ln2 C3 , LnC, Ln2 C, and Ln3 C phases are known (depending upon the lanthanide in question
and the conditions of synthesis). LnC2 adopt the CaC2 structure, containing isolated C2−
2
ions.

3.6 Nitrides
These can be made by direct synthesis from the elements at 1000 ◦ C. They have the NaCl
structure and can be hydrolysed to NH3 .

3.7 Hydrides
Ternary hydrides such as LaNi5 H6 have attracted attention as materials for electrodes in fuel
cells and for gas storage. Nickel/metal hydride batteries for notebook PCs are a less toxic alternative than Ni/Cd batteries; when charging, hydrogen generated at the negative electrode
enters the lattice of the La/Ni alloy (in practice a material such as La0.8 Nd0.2 Ni2.5 Co2.4 Si0.1
is used to improve corrosion resistance, storage capacity, discharge rate, etc.). The overall
cell reaction is:
LaNi5 + 6 Ni(OH)2 
 LaNi5 H6 + 6 NiOOH

32

The Lanthanide Elements and Simple Binary Compounds
The lanthanides also form simple binary hydrides on combination of the elements at about
300 ◦ C. These compounds have ideal compositions of MH2 and MH3 , but are frequently
non-stoichiometric. Thus lutetium forms phases with ranges LuH1.83 to LuH2.23 and LuH2.78
to LuH3.00 . Reaction of ytterbium with hydrogen under pressure gives YbH2.67 . MH3 are
obtained only at higher gas pressures, whilst europium, the lanthanide with the most stable
+2 state, forms only EuH2 . The dihydrides are generally good electrical conductors, and
thus are thought to be M3+ (H− )2 (e− ), whilst the trihydrides are salt-like nonconductors
believed to be M3+ (H− )3 . The hydrides are reactive solids, owing to the presence of the
easily hydrolysed H− ion.

3.8 Sulfides
These are quite important compounds. A number of stoichiometries exist, the most important
being Ln2 S3 . These can be made by direct synthesis, heating the elements together, or by
passing H2 S over heated LnCl3 . Eu2 S3 cannot be prepared by this latter route.
A range of compositions between Ln2 S3 and Ln3 S4 , the latter formed when Ln2 S3 lose
sulfur on heating, generally occurs. Ln2 S3 are insulators, genuine Ln(III) compounds, but
Ln3 S4 (having the Th3 P4 structure) are more complex. Some, like Ce3 S4 , are metallic conductors and are thus (Ln3+ )3 (S2− )4 (e− ); others, like Eu3 S4 and Sm3 S4 , are semiconductors
and may be (Ln2+ )(Ln3+ )2 (S2− )4 . The structures of Ln2 S3 fall into a pattern. La2 S3 to Dy2 S3
adopt the ‘Gd2 S3 ’ structure with 7-coordinate lanthanide ions; Dy2 S3 to Tm2 S3 have the
‘Ho2 S3 ’ structure with 6- and 7-coordinate lanthanides, whilst Yb2 S3 and Lu2 S3 have the
corundum structure with just 6 coordination.
Monsulfides MS are formed by direct combination. They adopt the NaCl structure but
have a variety of bonding types. YbS and EuS are genuine Ln2+ S2− but CeS exhibits the
magnetic properties expected for Ce3+ and has a bronze metallic lustre as well, so it thought
to be (Ce3+ )(S2− )(e− ). This substance not only has electrical conductivity in the metallic
region but also can be machined like a metal too. SmS has some unusual properties; it is
usually obtained as a black semiconducting phase, Sm2+ S2− , but can reportedly be turned
into a golden metallic phase by the action of pressure, polishing or even scratching on single
crystals.
Oxysulfides are also rather important. Y2 O2 S is used as a host material