The Chemistry of Organic Silicon Compounds. Volume 3
Edited by Zvi Rappoport and Yitzhak Apeloig
Copyright  2001 John Wiley & Sons, Ltd.
ISBN: 0-471-62384-9

The chemistry of
organic silicon compounds
Volume 3

i


THE CHEMISTRY OF FUNCTIONAL GROUPS
A series of advanced treatises founded by Professor
Saul Patai and under the general editorship of Professor Zvi Rappoport
The chemistry of alkenes (2 volumes)
The chemistry of the carbonyl group (2 volumes)
The chemistry of the ether linkage
The chemistry of the amino group
The chemistry of the nitro and nitroso groups (2 parts)
The chemistry of carboxylic acids and esters
The chemistry of the carbon – nitrogen double bond
The chemistry of amides
The chemistry of the cyano group
The chemistry of the hydroxyl group (2 parts)
The chemistry of the azido group
The chemistry of acyl halides
The chemistry of the carbon – halogen bond (2 parts)
The chemistry of the quinonoid compounds (2 volumes, 4 parts)
The chemistry of the thiol group (2 parts)
The chemistry of the hydrazo, azo and azoxy groups (2 volumes, 3 parts)

The chemistry of amidines and imidates (2 volumes)
The chemistry of cyanates and their thio derivatives (2 parts)
The chemistry of diazonium and diazo groups (2 parts)
The chemistry of the carbon – carbon triple bond (2 parts)
The chemistry of ketenes, allenes and related compounds (2 parts)
The chemistry of the sulphonium group (2 parts)
Supplement A: The chemistry of double-bonded functional groups (3 volumes, 6 parts)
Supplement B: The chemistry of acid derivatives (2 volumes, 4 parts)
Supplement C: The chemistry of triple-bonded functional groups (2 volumes, 3 parts)
Supplement D: The chemistry of halides, pseudo-halides and azides (2 volumes, 4 parts)
Supplement E: The chemistry of ethers, crown ethers, hydroxyl groups and their sulphur analogues (2
volumes, 3 parts)
Supplement F: The chemistry of amino, nitroso and nitro compounds and their derivatives
(2 volumes, 4 parts)
The chemistry of the metal – carbon bond (5 volumes)
The chemistry of peroxides
The chemistry of organic selenium and tellurium compounds (2 volumes)
The chemistry of the cyclopropyl group (2 volumes, 3 parts)
The chemistry of sulphones and sulphoxides
The chemistry of organic silicon compounds (3 volumes, 6 parts)
The chemistry of enones (2 parts)
The chemistry of sulphinic acids, esters and their derivatives
The chemistry of sulphenic acids and their derivatives
The chemistry of enols
The chemistry of organophosphorus compounds (4 volumes)
The chemistry of sulphonic acids, esters and their derivatives
The chemistry of alkanes and cycloalkanes
Supplement S: The chemistry of sulphur-containing functional groups
The chemistry of organic arsenic, antimony and bismuth compounds
The chemistry of enamines (2 parts)

The chemistry of organic germanium, tin and lead compounds
The chemistry of dienes and polyenes (2 volumes)
The chemistry of organic derivatives of gold and silver
UPDATES
The chemistry of ˛-haloketones, ˛-haloaldehydes and ˛-haloimines
Nitrones, nitronates and nitroxides
Crown ethers and analogs
Cyclopropane derived reactive intermediates
Synthesis of carboxylic acids, esters and their derivatives
The silicon – heteroatom bond
Synthesis of lactones and lactams
Syntheses of sulphones, sulphoxides and cyclic sulphides
Patai’s 1992 guide to the chemistry of functional groups — Saul Patai

C

Si

Si

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X


The chemistry of
organic silicon compounds
Volume 3


Edited by
ZVI RAPPOPORT
The Hebrew University, Jerusalem
and
YITZHAK APELOIG
Israel Institute of Technology, Haifa

2001
JOHN WILEY & SONS, LTD
CHICHESTER – NEW YORK – WEINHEIM – BRISBANE – SINGAPORE – TORONTO

An Interscience Publication
iii

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Copyright  2001 John Wiley & Sons, Ltd,
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Library of Congress Cataloging-in-Publication Data
The chemistry of organic silicon compounds / edited by Zvi Rappoport, Yitzhak Apeloig.
p. cm. — (The chemistry of functional groups. Supplement; Si)
Includes bibliographical references and index.
ISBN 0-471-62384-9 (alk. paper)
1. Organosilicon compounds. I. Rappoport, Zvi. II. Apeloig, Yitzhak. III. Series.
QD305.S54 C48 2001
00-043917
5470 .08 — dc21

British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British Library
ISBN 0 471 62384 9
Typeset in 9/10pt Times by Laser Words, Madras, India.
Printed and bound in Great Britain by Biddles Ltd, Guildford, Surrey.
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.

iv

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To

Noa, Nimrod
and

Naama

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Contributing authors
Yitzhak Apeloig

H. Bock

Bruno Boury

C. Chatgilialoglu
Cheol Ho Choi
Robert J. P. Corriu


Simonetta Fornarini

Mark S. Gordon
U. Herzog

Takeaki Iwamoto
Bettina Jaschke
Jurgen
Kapp
ă

Miriam Karni

Department of Chemistry, and the lise Meitner Minerva
Center for Computational Quantum Chemistry,
Technion-Israel Institute of Technology, Haifa 32000,
Israel
Institute of Inorganic Chemistry, Johann Wolfgang Goethe
University, Marie-Curie Street 11, D-60439 Frankfurt am
Main, Germany
Laboratoire de Chimie Mol´eculaire et Organisation du
Solide, UMR 5637, Universit´e de Montpellier II, CC007,
Place E. Bataillon, 34095 Montpellier cedex, France
I.Co.C.E.A., Consiglio Nazionale delle Ricerche, Via P.
Gobetti 101, 40129 Bologna, Italy
Department of Chemistry, Iowa State University, Ames,
Iowa 50011, USA
Laboratoire de Chimie Mol´eculaire et Organisation du
Solide, UMR 5637, Universit´e de Montpellier II, CC007,
Place E. Bataillon, 34095 Montpellier cedex, France

Dipartimento di Studi di Chimica e Tecnologia delle
Sostanze Biologicamente Attive, Universit´a di Roma ‘La
Sapienza’, P.le A. Moro 5, I-00185 Roma, Italy
Department of Chemistry, Iowa State University, Ames,
Iowa 50011, USA
Institute of Inorganic Chemistry, Freiberg University of
Mining and Technology, Leipziger Strasse 29, D-09596,
Freiberg, Germany
Department of Chemistry, Graduate School of Science,
Tohoku University, Aoba-ku, Sendai 980-8578, Japan
Institute of Inorganic Chemistry, University of Goettingen,
Tammannstrasse 4, D-37077 Goettingen, Germany
Computer Chemistry Center of the Institute of Organic
Chemistry, The University of Erlangen-Năurnberg,
Henkestrasse 42, 91054 Erlangen, Germany
Department of Chemistry, and the lise Meitner Minerva
Center for Computational Quantum Chemistry,
Technion-Israel Institute of Technology, Haifa 32000,
Israel
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viii
Mitsuo Kira
Uwe Klingebiel
William J. Leigh
Paul D. Lickiss
Tracy L. Morkin

Daniel E. Morse

Peter Neugebauer
Thomas R. Owens
C. H. Scheisser
Paul von Rague´ Schleyer
Jan Schraml

B. Solouki

David Y. Son
Kohei Tamao
Kozo Toyota
Manfred Weidenbruch
Robert West
Shigehiro Yamaguchi
Masaaki Yoshifuji

Contributing authors
Department of Chemistry, Graduate School of Science,
Tohoku University, Aoba-ku, Sendai 980-8578, Japan
Institute of Inorganic Chemistry, University of Goettingen,
Tammannstrasse 4, D-37077 Goettingen, Germany
Department of Chemistry, McMaster University, 1280
Main Street West, Hamilton, Ontario, L8S 4M1, Canada
Department of Chemistry, Imperial College of Science,
Technology and Medicine, London SW7 2AY, UK
Department of Chemistry, McMaster University, 1280
Main Street West, Hamilton, Ontario, L8S 4M1, Canada
Marine Biotechnology Center and Department of

Molecular, Cellular and Developmental Biology,
University of California, Santa Barbara, California 93106,
USA
Institute of Inorganic Chemistry, University of Goettingen,
Tammannstrasse 4, D-37077 Goettingen, Germany
Department of Chemistry, McMaster University, 1280
Main Street West, Hamilton, Ontario, L8S 4M1, Canada
School of Chemistry, University of Melbourne, Victoria
3010, Australia
Centre for Computational Quantum Chemistry, University
of Georgia, Athens, GA 30602-2525, USA
Institute of Chemical Process Fundamentals, Academy of
Sciences of the Czech Republic, 165 02 Prague, Czech
Republic
Institute of Inorganic Chemistry, Johann Wolfgang Goethe
University, Marie-Curie Street 11, D-60439 Frankfurt am
Main, Germany
Department of Chemistry, P.O. Box 750314, Southern
Methodist University, Dallas, Texas 75275-0314, USA
Institute for Chemical Research, Kyoto University, Uji,
Kyoto 611-0011, Japan
Department of Chemistry, Graduate School of Science,
Tohoku University, Aoba-ku, Sendai 980-8578, Japan
Fachbereich Chemie, Universităat Oldenburg, D-26111
Oldenburg, Germany
Organosilicon Research Center, Department of Chemistry,
University of Wisconsin, Madison, WI 53706, USA
Institute for Chemical Research, Kyoto University, Uji,
Kyoto 611-0011, Japan
Department of Chemistry, Graduate School of Science,

Tohoku University, Aoba-ku, Sendai 980-8578, Japan

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Foreword
The preceding volume on The chemistry of organic silicon compounds (Vol. 2) in ‘The
Chemistry of Functional Groups’ series (Z. Rappoport and Y. Apeloig, Eds.) appeared in
1998. It followed an earlier volume with the same title (S. Patai and Z. Rappoport, Eds.)
published in 1989 (now referred to as Vol. 1) and an update volume The silicon-heteroatom
bond in 1991. The appearance of the present volume only three years after the three
parts of Vol. 2 reflects the continuing rapid growth of many sub-fields of organosilicon
compounds and their chemistry.
The volume covers three types of chapters. First, the majority are new chapters,
including those which were planned but did not appear in Vol. 2 which we promised then to
include in a future volume. These include a comparison of the chemistry of organosilicon
compounds with that of their heavier group congeners, photoelectron spectroscopy
(which was covered in Vol. 1), silyl migrations, polysilanes, polysilanols, polysiloles,
organosilicon halides, nanostructured hybrid organic-inorganic solids, chemistry on
silicon surfaces, silicon based dendrimers and star compounds, synthesis of multiplybonded silicon-phosphorus compounds and a chapter on a biotechnological approach to
polysilsesquioxanes.
Second, chapters on topics which were covered incompletely or partially in Vol. 2 were
extended here by including new sub-topics related to the same themes. These include 29 Si
NMR, ion-molecule reactions of silicon ions and the reactivity of multiply-bonded silicon
compounds.
Finally, the rapid developments in recent years led to chapters which are updates of
those in Vol. 2. These include recent developments in the chemistry of silyl radicals, of
silicon-silicon multiple bonds and of silicon-nitrogen bonds.
The literature coverage in the book is mostly up to mid- or late-2000.
Two originally planned chapters, on the interplay between theory and experiments on

silicon and on silsesquioxanes, did not materialize, although these topics are covered
partially in other chapters. We hope to include these chapters in a future volume.
The chapters in this volume were written by authors from nine countries, thus reflecting
the international research activity in the chemistry of organosilicon compounds. We are
grateful to the authors for their contributions and we hope that this volume together
with its predecessor will serve as a major reference to the chemistry of organosilicon
compounds in the last decades.
We will be grateful to readers who draw our attention to mistakes in the present volume
and to those who mention new topics which deserve to be included in a future volume
on organosilicon compounds.

Jerusalem and Haifa
March 2001

ZVI RAPPOPORT
YITZHAK APELOIG
ix

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The Chemistry of Functional Groups
Preface to the series
The series ‘The Chemistry of Functional Groups’ was originally planned to cover in
each volume all aspects of the chemistry of one of the important functional groups in
organic chemistry. The emphasis is laid on the preparation, properties and reactions of the
functional group treated and on the effects which it exerts both in the immediate vicinity
of the group in question and in the whole molecule.
A voluntary restriction on the treatment of the various functional groups in these
volumes is that material included in easily and generally available secondary or tertiary sources, such as Chemical Reviews, Quarterly Reviews, Organic Reactions, various

‘Advances’ and ‘Progress’ series and in textbooks (i.e. in books which are usually found
in the chemical libraries of most universities and research institutes), should not, as a rule,
be repeated in detail, unless it is necessary for the balanced treatment of the topic. Therefore each of the authors is asked not to give an encyclopaedic coverage of his subject,
but to concentrate on the most important recent developments and mainly on material that
has not been adequately covered by reviews or other secondary sources by the time of
writing of the chapter, and to address himself to a reader who is assumed to be at a fairly
advanced postgraduate level.
It is realized that no plan can be devised for a volume that would give a complete coverage of the field with no overlap between chapters, while at the same time preserving the
readability of the text. The Editors set themselves the goal of attaining reasonable coverage
with moderate overlap, with a minimum of cross-references between the chapters. In this
manner, sufficient freedom is given to the authors to produce readable quasi-monographic
chapters.
The general plan of each volume includes the following main sections:
(a) An introductory chapter deals with the general and theoretical aspects of the group.
(b) Chapters discuss the characterization and characteristics of the functional groups,
i.e. qualitative and quantitative methods of determination including chemical and physical
methods, MS, UV, IR, NMR, ESR and PES — as well as activating and directive effects
exerted by the group, and its basicity, acidity and complex-forming ability.
(c) One or more chapters deal with the formation of the functional group in question,
either from other groups already present in the molecule or by introducing the new group
directly or indirectly. This is usually followed by a description of the synthetic uses of
the group, including its reactions, transformations and rearrangements.
(d) Additional chapters deal with special topics such as electrochemistry, photochemistry, radiation chemistry, thermochemistry, syntheses and uses of isotopically labelled
compounds, as well as with biochemistry, pharmacology and toxicology. Whenever applicable, unique chapters relevant only to single functional groups are also included (e.g.
‘Polyethers’, ‘Tetraaminoethylenes’ or ‘Siloxanes’).
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xii

Preface to the series

This plan entails that the breadth, depth and thought-provoking nature of each chapter
will differ with the views and inclinations of the authors and the presentation will necessarily be somewhat uneven. Moreover, a serious problem is caused by authors who deliver
their manuscript late or not at all. In order to overcome this problem at least to some
extent, some volumes may be published without giving consideration to the originally
planned logical order of the chapters.
Since the beginning of the Series in 1964, two main developments have occurred.
The first of these is the publication of supplementary volumes which contain material
relating to several kindred functional groups (Supplements A, B, C, D, E, F and S). The
second ramification is the publication of a series of ‘Updates’, which contain in each
volume selected and related chapters, reprinted in the original form in which they were
published, together with an extensive updating of the subjects, if possible, by the authors
of the original chapters. A complete list of all above mentioned volumes published to
date will be found on the page opposite the inner title page of this book. Unfortunately,
the publication of the ‘Updates’ has been discontinued for economic reasons.
Advice or criticism regarding the plan and execution of this series will be welcomed
by the Editors.
The publication of this series would never have been started, let alone continued,
without the support of many persons in Israel and overseas, including colleagues, friends
and family. The efficient and patient co-operation of staff-members of the publisher also
rendered us invaluable aid. Our sincere thanks are due to all of them.
SAUL PATAI
ZVI RAPPOPORT

The Hebrew University
Jerusalem, Israel


Sadly, Saul Patai who founded ‘The Chemistry of Functional Groups’ series died in
1998, just after we started to work on the 100th volume of the series. As a long-term
collaborator and co-editor of many volumes of the series, I undertook the editorship and
this is the third volume to be edited since Saul Patai passed away. I plan to continue
editing the series along the same lines that served for the first hundred volumes and I
hope that the continuing series will be a living memorial to its founder.
ZVI RAPPOPORT

The Hebrew University
Jerusalem, Israel
May 2000

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Contents
1 Theoretical aspects of compounds containing Si, Ge, Sn and Pb
Mirian Karni, Jurgen
Kapp, Paul von Rague Schleyer and
ă
Yitzhak Apeloig
2 (Helium I)-photoelectron spectra of silicon compounds: History
and achievements concerning their molecular states
H. Bock and B. Solouki
3

29

Si NMR experiments in solutions of organosilicon compounds
Jan Schraml


1

165

223

4 Silyl radicals
C. Chatgilialoglu and C. H. Schiesser

341

5 Recent advances in the chemistry of silicon – silicon multiple
bonds
Manfred Weidenbruch

391

6 Recent developments in the chemistry of compounds with
silicon-nitrogen bonds
Peter Neugebauer, Bettina Jaschke and Uwe Klingebiel

429

7 Organosilicon halides — synthesis and properties
Uwe Herzog

469

8 Synthesis of multiply bonded phosphorus compounds using

silylphosphines and silylphosphides
Masaaki Yoshifuji and Kozo Toyota

491

9 Polysilanes: Conformations, chromotropism and conductivity
Robert West

541

10 Nanostructured hybrid organic – inorganic solids. From molecules
to materials
Bruno Boury and Robert J. P. Corriu

565

11 Polysiloles and related silole-containing polymers
Shigehiro Yamaguchi and Kohei Tamao

641

12 Polysilanols
Paul D. Lickiss

695

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xiv

Contents

13 Silicon-based dendrimers and hyperbranched polymers
David Y. Son

745

14 Biotechnology reveals new routes to synthesis and structural
control of silica and polysilsesquioxanes
Daniel E. Morse

805

15 Chemistry on silicon surfaces
Cheol Ho Choi and Mark S. Gordon

821

16 Silyl migrations
Mitsuo Kira and Takeaki Iwamoto

853

17 Kinetic studies of the reactions of SiDC and SiDSi bonds
Tracy L. Morkin, Thomas R. Owens and William J. Leigh

949


18 Ion-molecule reactions of silicon cations
Simonetta Fornarini

1027

Author index

1059

Subject index

1127

Contents of Volume 1
Contents of Volume 2

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List of abbreviations used
Ac
acac
Ad
AIBN
Alk
All
An
Ar


acetyl (MeCO)
acetylacetone
adamantyl
azoisobutyronitrile
alkyl
allyl
anisyl
aryl

Bn
Bz
Bu

benzyl
benzoyl C6 H5 CO
butyl (also t-Bu or But )

CD
CI
CIDNP
CNDO
Cp
CpŁ

circular dichroism
chemical ionization
chemically induced dynamic nuclear polarization
complete neglect of differential overlap
Á5 -cyclopentadienyl
Á5 -pentamethylcyclopentadienyl


DABCO
DBN
DBU
DIBAH
DME
DMF
DMSO

1,4-diazabicyclo[2.2.2]octane
1,5-diazabicyclo[4.3.0]non-5-ene
1,8-diazabicyclo[5.4.0]undec-7-ene
diisobutylaluminium hydride
1,2-dimethoxyethane
N,N-dimethylformamide
dimethyl sulphoxide

ee
EI
ESCA
ESR
Et
eV

enantiomeric excess
electron impact
electron spectroscopy for chemical analysis
electron spin resonance
ethyl
electron volt


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xvi

List of abbreviations used

Fc
FD
FI
FT
Fu

ferrocenyl
field desorption
field ionization
Fourier transform
furyl(OC4 H3 )

GLC

gas liquid chromatography

Hex
c-Hex
HMPA
HOMO

HPLC

hexyl(C6 H13 )
cyclohexyl(C6 H11 )
hexamethylphosphortriamide
highest occupied molecular orbital
high performance liquid chromatography

iIp
IR
ICR

iso
ionization potential
infrared
ion cyclotron resonance

LAH
LCAO
LDA
LUMO

lithium aluminium hydride
linear combination of atomic orbitals
lithium diisopropylamide
lowest unoccupied molecular orbital

M
M
MCPBA

Me
MNDO
MS

metal
parent molecule
m-chloroperbenzoic acid
methyl
modified neglect of diatomic overlap
mass spectrum

n
Naph
NBS
NCS
NMR

normal
naphthyl
N-bromosuccinimide
N-chlorosuccinimide
nuclear magnetic resonance

Pc
Pen
Pip
Ph
ppm
Pr
PTC

Pyr

phthalocyanine
pentyl(C5 H11 )
piperidyl(C5 H10 N)
phenyl
parts per million
propyl (also i-Pr or Pri )
phase transfer catalysis or phase transfer conditions
pyridyl (C5 H4 N)

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List of abbreviations used
R
RT

any radical
room temperature

sSET
SOMO

secondary
single electron transfer
singly occupied molecular orbital

tTCNE
TFA

THF
Thi
TLC
TMEDA
TMS
Tol
Tos or Ts
Trityl

tertiary
tetracyanoethylene
trifluoroacetic acid
tetrahydrofuran
thienyl(SC4 H3 )
thin layer chromatography
tetramethylethylene diamine
trimethylsilyl or tetramethylsilane
tolyl(MeC6 H4 )
tosyl(p-toluenesulphonyl)
triphenylmethyl(Ph3 C)

Xyl

xylyl(Me2 C6 H3 )

xvii

In addition, entries in the ‘List of Radical Names’ in IUPAC Nomenclature of Organic
Chemistry, 1979 Edition, Pergamon Press, Oxford, 1979, p. 305 –322, will also be used
in their unabbreviated forms, both in the text and in formulae instead of explicitly drawn

structures.

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The Chemistry of Organic Silicon Compounds. Volume 3
Edited by Zvi Rappoport and Yitzhak Apeloig
Copyright  2001 John Wiley & Sons, Ltd.
ISBN: 0-471-62384-9

CHAPTER 1

Theoretical aspects of compounds
containing Si, Ge, Sn and Pb
MIRIAM KARNI and YITZHAK APELOIG
Department of Chemistry, and the Lise Meitner Minerva Center for Computational
Quantum Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel
Fax: 972-4-8294601; email: and

and

ă
JURGEN
KAPP and PAUL VON R. SCHLEYERa
Computer Chemistry Center of the Institute of Organic Chemistry, The University of
Erlangen-Nurnberg,
Henkestrasse 42, 91054 Erlangen, Germany
ă
a
Current address: Center for Computational Quantum Chemistry, University of

Georgia, Athens, GA 30602-2525, USA
Fax: 706 542-7514; email:

I. LIST OF ABBREVIATIONS . . . . . . . . . . . . . . . . . . .
II. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . .
III. PERIODIC TRENDS IN THE PROPERTIES OF GROUP
14 ELEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. Radial Orbital Extensions . . . . . . . . . . . . . . . . . . .
B. Relativistic Effects . . . . . . . . . . . . . . . . . . . . . . .
C. Hybridization . . . . . . . . . . . . . . . . . . . . . . . . . . .
D. Electronegativity and Bonding . . . . . . . . . . . . . . . .
E. Spin –Orbit Coupling . . . . . . . . . . . . . . . . . . . . . .
F. The Role of d Orbitals . . . . . . . . . . . . . . . . . . . . .
IV. THEORETICAL METHODS . . . . . . . . . . . . . . . . . . .
A. Nonrelativistic Theoretical Methods . . . . . . . . . . . .
B. Relativistic Methods . . . . . . . . . . . . . . . . . . . . . .
C. Effective Core Potential Basis Sets . . . . . . . . . . . . .
D. Methods for Analysis of the Electronic Structure . . .

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14


2

Miriam Karni, Yitzhak Apeloig, Jăurgen Kapp and Paul von R. Schleyer
V. SINGLY BONDED COMPOUNDS . . . . . . . . . . . . . . . . . . . . . . . . .
A. MH4 (metallanes) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Geometries, ionization potentials and nuclear spin –spin
couplings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Bond energies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3. The stability of MH4 relative to MH2 C H2 . . . . . . . . . . . . . . . .
4. Charged MH4 species: MH4 C and MH4 2C . . . . . . . . . . . . . . . .
B. Mono-substituted Singly-bonded MH3 R Metallanes . . . . . . . . . . . . .
1. General trends in the M R bond dissociation energies . . . . . . . . .
2. MH3 R, R D halogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
a. Geometries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
b. M R bond dissociation energies . . . . . . . . . . . . . . . . . . . . .
3. Ethane analogs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
a. Geometries and rotational barriers . . . . . . . . . . . . . . . . . . . .
b. Bond dissociation energies . . . . . . . . . . . . . . . . . . . . . . . . .
c. Nonclassical bridged structures of ethane analogs . . . . . . . . . .
4. Classical linear Mn H2nC2 chains . . . . . . . . . . . . . . . . . . . . . . .
C. Multiply-substituted Singly-bonded Compounds . . . . . . . . . . . . . . .
1. M(CH3 )4 and MX4 , X D halogen . . . . . . . . . . . . . . . . . . . . . .
2. (CH3 )n MX4 n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3. Relative stabilities of MIV and MII compounds . . . . . . . . . . . . . .
4. Oxides and sulfides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5. MLi4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

D. Hypercoordinated Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. MH5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. MX5 , X D alkali metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3. MX6 , MX7 and MX8 , X D alkali metals . . . . . . . . . . . . . . . . . .
E. Cyclic Metallanes: Rings, Polycyclic and Polyhedral Compounds . . . .
1. Saturated ring compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . .
a. c-M3 H6 and c-M4 H8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
i. Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ii. Strain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
iii. Stability towards cleavage . . . . . . . . . . . . . . . . . . . . . . .
iv. Other structures of 3-MRs . . . . . . . . . . . . . . . . . . . . . . .
b. Metalloles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
c. Heterocyclic 3- and 4-membered rings . . . . . . . . . . . . . . . . .
i. 3-MRs, c-(R2 M)2 X . . . . . . . . . . . . . . . . . . . . . . . . . . .
ii. 1,3-Dioxa-2,4-dimetaletanes, c-(MH2 )2 O2 . . . . . . . . . . . . .
2. Polycyclic and polyhedral metallanes . . . . . . . . . . . . . . . . . . . .
a. Bicyclic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
b. Propellanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
c. Polyhedral cage compounds: tetrahedrane, prismane, cubane
and larger Mn Hn systems . . . . . . . . . . . . . . . . . . . . . . . . . .
d. Polyhedral metallaboranes . . . . . . . . . . . . . . . . . . . . . . . . . .
VI. MULTIPLY-BONDED SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . .
A. Historical Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. MDM0 Doubly-bonded Compounds (Metallenes) . . . . . . . . . . . . . . .
1. Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. The double-bond strength . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3. Stability relative to isomeric structures . . . . . . . . . . . . . . . . . . .

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1. Theoretical aspects of compounds containing Si, Ge, Sn and Pb

VII.

VIII.
IX.
X.

a. Relative to the corresponding metallylenes . . . . . . . .
b. Relative to bridged isomers . . . . . . . . . . . . . . . . . .

i. Hydrogen bridged isomers . . . . . . . . . . . . . . . .
ii. -Donor bridged isomers . . . . . . . . . . . . . . . . .
C. R2 MDX Compounds . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Structures and bond energies . . . . . . . . . . . . . . . . . . .
2. Isomerization to metallylenes . . . . . . . . . . . . . . . . . .
D. Increasing the Number of Double Bonds . . . . . . . . . . . . .
1. Heavier analogs of 1,3-butadiene . . . . . . . . . . . . . . . .
2. Heavier analogs of allene . . . . . . . . . . . . . . . . . . . . .
E. Triply-bonded Metallenes, RMÁM0 R0 . . . . . . . . . . . . . . .
1. Structures and bond nature . . . . . . . . . . . . . . . . . . . .
2. Potential energy surfaces . . . . . . . . . . . . . . . . . . . . .
a. HCÁMH, M D Si, Ge . . . . . . . . . . . . . . . . . . . . .
b. HMÁMH and HMÁM0 H . . . . . . . . . . . . . . . . . . .
F. Aromatic compounds . . . . . . . . . . . . . . . . . . . . . . . . . .
1. The congeners of benzene and their isomers . . . . . . . . .
2. Metallacyclopropenium cations . . . . . . . . . . . . . . . . .
3. Metallacyclopentadienyl anion and dianion . . . . . . . . .
4. Metallocenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REACTIVE INTERMEDIATES . . . . . . . . . . . . . . . . . . . . .
A. Divalent Compounds (Metallylenes) . . . . . . . . . . . . . . . .
1. MH2 and MX2 (X D halogen) . . . . . . . . . . . . . . . . . .
2. Stable metallylenes . . . . . . . . . . . . . . . . . . . . . . . . .
3. Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
a. 1,2-Hydrogen shifts . . . . . . . . . . . . . . . . . . . . . . .
b. Insertion and addition reactions . . . . . . . . . . . . . . .
i. Insertion into H2 . . . . . . . . . . . . . . . . . . . . . .
ii. Insertion into M H bonds . . . . . . . . . . . . . . . .
iii. Insertion into X H -bonds . . . . . . . . . . . . . .
iv. Addition to double bonds . . . . . . . . . . . . . . . .
v. Addition to acetylene . . . . . . . . . . . . . . . . . . .

B. Tricoordinated Compounds . . . . . . . . . . . . . . . . . . . . . .
1. Tricoordinated cations . . . . . . . . . . . . . . . . . . . . . . .
a. Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
b. Thermodynamic and kinetic stability of MR3 C cations
2. Tricoordinated radicals . . . . . . . . . . . . . . . . . . . . . . .
3. Tricoordinated anions . . . . . . . . . . . . . . . . . . . . . . .
C. Pentacoordinated Compounds . . . . . . . . . . . . . . . . . . . .
1. Pentacoordinated cations . . . . . . . . . . . . . . . . . . . . .
2. Pentacoordinated radicals . . . . . . . . . . . . . . . . . . . . .
3. Pentacoordinated anions . . . . . . . . . . . . . . . . . . . . . .
CONCLUSIONS AND OUTLOOK . . . . . . . . . . . . . . . . . .
ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . .
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ARPP
ASE

I. LIST OF ABBREVIATIONS
averaged relativistic pseudopotential
aromatic stabilization energy

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147


4

Miriam Karni, Yitzhak Apeloig, Jăurgen Kapp and Paul von R. Schleyer

B3LYP

Becke’s 3-parameter hybrid with Lee, Young and Parr’s correlation
functional
BLYP
B88 exchange functional with Lee, Young and Parr’s correlation functional
CASSCF
complete active space SCF
CCSD(T)
coupled cluster with single and double excitations (followed by a perturbation treatment of triple excitations)
CEP
compact effective potential
CI
configuration interaction
CIDVD
configuration interaction calculation including single and double substitutions
with the contribution of quadruple excitations estimated with Davidson’s
formula
CISD
configuration interaction calculations including single and double substitutions
Dep
2,6-diethylphenyl
DFT

density functional theory
DHF
Dirac Hartree – Fock
Dip
2,6-diisopropylphenyl
Dmp
2,6-dimethylphenyl
DSSE
divalent state stabilization energy
DZCd
double-zeta quality basis set augmented with polarization functions on nonhydrogen atoms
DZP
double-zeta quality basis set augmented with polarization functions on all
atoms
ECP
effective core potential
LANL1DZ Los Alamos ECP C double-zeta quality basis set
LDA
local density approximation
LSDA
local spin density approximation
Mes
mesityl (2,4,6-trimethylphenyl)
MNDO
modified neglect of diatomic overlap
MPn
Møller –Plesset perturbation method of the nth order
MRSDCI
multireference singles C doubles configuration interaction
MRSOCI

multireference second order configuration interaction
NAO
natural atomic orbital
NBO
natural bond orbital analysis
NICS
nucleus independent chemical shift
NLMO
natural localized molecular orbital
NPA
natural population analysis
NRT
natural resonance theory
PM3
modified neglect of diatomic overlap –parametric method number 3
PSP
pseudopotential
PT
perturbation theory
QCISD
quadratic configuration interaction calculations including single and double
substitutions
QCISD(T) quadratic configuration interaction calculations including single and double
substitutions with the addition of triples contribution to the energy
RCEP
relativistic compact effective potential
RECP
relativistic effective core potential
SCF
self-consistent field

SDD
the Stuttgart/Dresden double-zeta effective core potential
SOC
spin –orbit coupling
SOCI
second order configuration interaction
Tbt
2,4,6-tris[bis(trimethylsilyl)methyl]phenyl

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1. Theoretical aspects of compounds containing Si, Ge, Sn and Pb
TCSCF
Tip
TMS
VDZ
VQZ

5

two-configuration self-consistent field
2,4,6-tris(isopropyl)phenyl
trimethylsilyl
valence double-zeta quality basis set
valence quadruple-zeta quality basis set
II. INTRODUCTION

It is difficult to select ‘the most important group of the Periodic Table of the Elements’ — but if such a choice has to be made group 14, consisting of carbon, silicon,
germanium, tin and lead, would have a good chance of being chosen. Carbon is of major

importance to life, silicon is the most abundant element in the earth’s crust and, jointly
with germanium, drives the computer revolution, while the metals, tin and lead, known
since antiquity, still continue to play an important role in science and technology. Numerous review articles deal with the different chemical and physical properties of carbon and
its congeners1 – 7 . It is now well accepted that the chemical behavior of the heavier main
group elements (not only those of group 14) should be described as ‘normal’, while that
of the first row, including the elements Li to Ne, is exceptional6 . A large gap in physical
properties and in chemical behavior is evident between carbon, the non-metal, and silicon the (semi-)metal, and this point has been discussed extensively in the literature3,6,7 .
However, it is a gross oversimplification to assume that the chemistry of the heavier
group 14 elements Ge, Sn and Pb resembles the chemistry of silicon. The known chemistry of silicon, germanium, tin and lead refute this assumption8 . Striking and surprising
changes down the group are observed, when compounds of the heavier congeners with
common functional groups are compared1 . Examples are double bonds and small strained
rings composed of group 14 metals. The review and analysis of the similarities and differences which occur when silicon is substituted by its heavier congeners is the focus of
this chapter. The review focuses on the contributions of theoretical studies, but important
experimental developments are also discussed briefly and the reader is directed to the
original references for further reading.
The experimental progress of the chemistry of compounds containing silicon and its
congeners during the last two decades has been spectacular1,9 – 24 . These developments
were paralleled by the extension of computational methods to the heavier elements25 .
Quantum mechanical calculations were extremely helpful in explaining the differences
between carbon and silicon chemistry and in directing some of the pioneering experimental work in silicon chemistry. The theoretical studies on silicon compounds were
reviewed extensively by Apeloig in 19897 . Reviews on the theoretical aspects of the
chemistry of specific groups of silicon compounds are also available, e.g. multiply-bonded
and divalent silicon compounds26,27 , aromatic and antiaromatic silicon compounds28a and
others4,28b – d,29 . However, considerably fewer theoretical studies dealt with compounds
of germanium and the heavier group 14 metals. This is not surprising, as reliable calculations of heavier elements required larger basis sets and more sophisticated theoretical
treatments, e.g. the inclusion of relativistic effects30 , and therefore much larger computer
capabilities. Consequently, most of the earlier theoretical surveys on group 14 compounds
included only compounds of carbon and silicon4,7,26 – 29 , and occasionally also germanium compounds. Nevertheless, calculations on small molecules like MH4 and MO with
all group 14 elements (M D C to Pb) date back to the early 1970s31 . The results of the
calculations on small molecules32 , for which sufficient experimental information (geometries, dipole moments etc.) was available for calibration33 , were used as benchmarks to


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6

Miriam Karni, Yitzhak Apeloig, Jăurgen Kapp and Paul von R. Schleyer

check the performance of new computational procedures, such as relativistic Dirac – Fock
calculations32 .
Larger systems can be calculated with more reasonable computer resources and time
requirements than required for all-electron basis sets, by employing effective core potentials (ECPs)34 . ECPs, which were refined mostly during the 1980s, replace the explicit
treatment of the core electrons (i.e. nonvalence electrons) by a suitable function. This
reduces dramatically the computer time required for a particular calculation. In addition,
most ECPs were fit to include also relativistic procedures34 and they thus introduce relativistic effects into formally nonrelativistic calculations. This is the reason why most
theoretical calculations on compounds of heavier group 14 elements are currently carried out using ECPs. ECPs are, of course, an approximation and many effects, e.g. core
polarization and correlation between core and valence electrons, are ignored. Errors can
therefore be expected to be larger than in full theoretical treatments. Nevertheless, the
advantages of ECPs override their disadvantages, making them very popular and widely
used. A more detailed discussion of the theoretical methods is given in Section IV. In
any event, the goal of most investigations on compounds of heavy group 14 elements
is not necessarly to achieve the highest possible accuracy but to gain insights, e.g., on
the variation of the structures and bonding when moving down the Periodic Table. Such
insight is indeed the major purpose of this chapter.
Unfortunately, experimental investigations can contribute relatively little to the calibration and testing of the theoretical calculations for the heavier group 14 elements, in
contrast to the very close theoretical –experimental calibration which is possible in carbon chemistry. Many basic systems, which can be calculated with a variety of theoretical
methods including very sophisticated ones, are in many cases unknown experimentally. In
addition, many of the group 14 compounds with unusual structures or properties, synthesized in the last decade, are stabilized by large bulky substituents1,2,9 – 24 . Therefore, their
experimental properties (structure, spectroscopy, reactivity) are often dominated by substituent effects and they do not necessarily represent the characteristic inherent behavior
of the parent compounds. Furthermore, many of these sterically crowded systems are too

large to be computed adequately, and hence, in many cases, the theoretical calculations
are performed for model systems and not for the actual experimental systems, making a
theoretical –experimental comparison difficult and sometimes even speculative as various
assumptions have to be made.
The main objective of this chapter is to compare compounds of silicon with compounds
of its heavier congeners, germanium, tin and lead. Therefore, we review mostly studies
which provide a comparison between at least silicon and one of the heavier congeners.
For completeness of the picture we mention in many cases also the behavior of the
corresponding carbon compounds.
We will discuss first the general trends that lead to the differences in behavior of
group 14 elements. Next, we discuss singly-bonded compounds of group 14 metals and
then multiply bonded systems, e.g. doubly-bonded analogs of ethylene and triply-bonded
analogs of acetylene. We continue with a discussion of aromatic systems, e.g. benzene
analogs, and complete the chapter with a discussion of reactive intermediates, mainly the
divalent carbene-like MR2 systems, charged species and radicals. We will also present a
short overview of the major different theoretical methods which were applied to calculate group 14 element compounds, so that experimentalists who are unfamiliar with the
theoretical methods, terms and jargon will be able to follow the discussion.
The amount of theoretical work done in this field in the last 15 years has been overwhelming. As the main purpose of this review is to provide insight and guidelines to
the similarities and differences between the compounds of group 14 elements, this review

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1. Theoretical aspects of compounds containing Si, Ge, Sn and Pb

7

is not comprehensive. We have concentrated on the systems from which we believe the
most important lessons can be learned.
III. PERIODIC TRENDS IN THE PROPERTIES OF GROUP 14 ELEMENTS

An understanding of the properties of the elements is the key to understanding the properties of their compounds.
Some important physical properties of group 14 elements are given in Table 135 – 44 .
A detailed comparison of the atomic properties of C and Si was given by Apeloig7 and
by Corey3 . A comparison of the important physical properties of all group 14 elements
was given by Basch and Hoz45 . Much of the discussion below is based on the landmark
review of Kutzelnigg which was published in 19846 .
TABLE 1.

Physical properties of group 14 elements

Electronegativity
Allred – Rochowa
Paulingb
Allenc
Atomic and ionic radii (pm)d
neutral
2C
4C
Valence orbital energy (eV)f
s
pg
Difference
r D rp rs ) (pm)h
Atomic spin – orbit
coupling (kcal mol 1 )i
Ionization energy (eV)j
nsk
npl
Electron affinity (eV)m
Hybridization of M in

MHn4

C

Si

Ge

Sn

Pb

2.50
2.55
2.54

1.74
1.90
1.92

2.02
2.01
1.99

1.72
1.96
1.82

1.55
2.33

—

77

118

16

40 – 42

121
73
53

140
93
69 – 71

175e
118 – 120
78 – 84

19.39
11.07
8.32
0.2
0.05

14.84
7.57

7.27
20.3
0.2

15.52
7.29
8.23
24.9
1.6

13.88
6.71
7.17
28.5
4.8

15.41
6.48
8.93
35.8
22.4

16.60
11.26
1.26
sp3.17

13.64
8.15
1.76

sp2.08

14.43
7.90
1.81
sp2.05

13.49
7.39
1.68
sp1.79

16.04
7.53
1.91
sp1.75

a From Reference 35a.
b From Reference 35b.
c From Reference 36.
d From References 37 – 39.
e Metallic distance.
f From Reference 40.
g Spin – orbit averaged.
h Difference of the orbital radii of maximal electron density between the valence p and s orbitals; from Reference 40.
i 3 P !3 P energy difference, see text; from Reference 41.
0
1
j Spin – orbit averaged; from Reference 42.
k For the process: ns2 np2 (3 P ! ns1 np2 (4 P).

l For the process: ns2 np2 3 P ! ns2 np1 (2 P).
m From Reference 43.
n According to NBO analysis, at B3LYP/TZCP; from Reference 44.

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8

Miriam Karni, Yitzhak Apeloig, Jăurgen Kapp and Paul von R. Schleyer

A. Radial Orbital Extensions

The changes of the radii (r) of the ns and np atomic orbitals of group 14 elements as a
function of the element are shown in Figure 1. It could have been expected that the radii
of the ns and the np orbitals would increase monotonically down the group because the
principal quantum number n increases. However, a zig-zag behavior, with an irregular
behavior for Ge and Pb, is actually found (Figure 1). This behavior is common to the
third- and fifth-row atoms of the Periodic Table. Thus, in C, the 2s orbital is relatively
extended, as a result of the repulsion of the 2s electrons by the 1s2 core electrons, while
the 2p orbitals which are not shielded by other p electrons are relatively contracted. In
silicon the radii of both the 3s and the 3p orbitals increase (due to the presence of 2s and
2p electrons), but the latter expand more than the former because now the 2p electrons
repel the 3p shell. Ge exhibits a break in the trend due to the imperfect screening by
the 3d10 shell which increases the effective nuclear charge for the 4s and 4p electrons.
This causes the 4s orbital to contract and, to a limited extent, the 4p orbital as well (the
so-called d-block contraction30 ). In Sn, the 5s and 5p orbitals increase in size. This is
followed by a drop in the size of the 6s orbital of Pb and less so for the 6p orbitals due
to the ‘lanthanide and relativistic contraction’30 (see below).
Of great importance is the radial orbital extension difference, r D rp rs , (Table 1

and Figure 1). Due to the orbital behavior described above, r for carbon is only
0.2 pm. However, r increases successively in a zig-zag fashion (caused by the d-block
160

140
p orbital

rmax (pm)

120

s orbital

100

80

60

C

Si

Ge

Sn

Pb

FIGURE 1. The calculated sizes of the valence ns and np orbitals of group 14 elements. Adapted

from Reference 5

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1. Theoretical aspects of compounds containing Si, Ge, Sn and Pb

9

contraction and relativistic effects) on moving down the Periodic Table. The largest r
is found for Pb and this contributes to the unique structures of Pb containing molecules
(Section V.C.2).
B. Relativistic Effects

As the nuclei become heavier, the strong attraction of the electrons by the very large
nuclear charge causes the electrons to move very rapidly and behave relativistically, i.e.
their relative mass (m) increases according to equation 1, and the effective Bohr radius
(a0 ) for inner electrons with large average speeds decreases according to equation 230 .
m D m0 / 1

v/c

1/2

1

In equation 1, m0 is the rest mass of the electron, v is the average electron speed and c is
the speed of light (137 au); 1 v/c 1/2 is the relativistic correction. The average speed
for a 1s electron at the nonrelativistic limit is Z au, where Z is the atomic number30 .
a0 D 4 ε0 h2 /me2


2

In equation 2, ε0 is the permittivity of free space and e is the charge on the electron.
According to equations 1 and 2 the relativistic 1s contractions of Ge, Sn and Pb are 3%,
8% and 20%, respectively30b . Because the higher shells have to be orthogonal to the lower
ones, the higher ns-orbitals will suffer similar contractions30a . The effect of relativity on
the np orbitals is smaller than for the ns orbitals, since the angular momentum keeps p
electrons away from the nucleus. The relativistic contraction of ns orbitals for the heavy
elements stabilizes them as shown in Figure 2, having the largest effect for Pb, where the
s – p energy difference of 8.93 eV is the largest in the series (Table 1).
−12.0
nonrelativistic energy

s orbital energy (eV)

−14.0

relativistic energy

−16.0

−18.0

−20.0

C

Si


Ge

Sn

Pb

FIGURE 2. Stabilization of the valence ns orbital due to the relativistic effect. Reproduced by
permission of Gordon and Breach Science Publishers from Reference 5

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10

Miriam Karni, Yitzhak Apeloig, Jăurgen Kapp and Paul von R. Schleyer

Wang and Schwarz have pointed out recently46 that although the direct influence of
the relativistic effect is in the vicinity of the nucleus, and thus is most important for the
core electrons, the orbitals of the valence s electrons and to a lesser extent also those
of the outer p electrons have ‘inner tails’ that penetrate the core. For this reason the
valence electrons also experience a direct relativistic effect. Thus, although the probability of a valence electron to be close to the nucleus is small, the relativistic effects
propagate to the outer valence shell and change also the energies of the valence orbitals.
The d and f orbitals are not core-penetrating and they experience indirect relativistic
destabilization, due to a more effective shielding by the contracted s and p shells, particularly those with the same quantum number as the d and f orbitals30,46 – 50 . The effects
of relativity on orbital energies and on their radial extension affects the excitation energies, ionization potentials, electron affinities, electronegativity and atom polarizability,
and through these properties influence the chemical bonding and reactivity of heavier
group 14 elements.
The effect of relativity on various properties (e.g. ionization energies, electron affinity
etc.) of the ‘eka-lead’ element 114 in comparison to the other group 14 elements was
studied recently by Schwerdtfeger and coworkers48b .

C. Hybridization

The major reason for the different structural behavior of compounds of the second
period of the Periodic Table (i.e. Li to F) and those of higher periods is the relative
radial extension of the valence s and p orbitals. For carbon, the radial extension of the
2s and 2p orbitals is almost the same (Figure 1). Thus hybridization, which requires the
promotion of an electron from 2s to 2p, is very effective. In contrast, the 3p, 4p and
higher period atomic orbitals are significantly ‘larger’ than the corresponding 3s, 4s and
higher period orbitals, and consequently r D rp rs increases when moving down group
14 (Table 1). Hybridization is thus more difficult for the heavier elements of the group.
This simple trend explains many striking phenomena in group 14 chemistry, such as,
the ‘inert s-pair effect’, which states that the pair of s electrons is ‘inert’ and only the p
electrons are employed in the bonding44,51 , and hence the preference of Pb (which has the
largest r) to form divalent PbR2 compounds rather than tetravalent PbR4 compounds
(see Section V.C.3).
Let us consider the two common oxidation states (II and IV) of group 14 elements.
In divalent compounds (oxidation state II), the valence s orbitals of the heavy elements,
the ‘inert pair’, are lone pairs with minor p contributions; the chemical bonds are formed
primarily by p orbitals (one p orbital remains empty). In tetravalent compounds of heavier
group 14 elements (Si ! Pb), the s-orbitals of the metal do contribute to the bonding53 .
However, as hybridization is less effective for these elements than for C52 , these elements have less tendency to form spn hybrids and they tend to keep the atomic s2 p2
valence hybridization. However, as there are 4 bonds to be made, the s orbitals of the
tetravalent group 14 metals (Si ! Pb) do contribute to the bonding53 . When localized
molecular orbitals are used, the spn hybridizations of M in MH4 show a strong decrease
in n along the series: C > Si > Sn > Pb (Table 1), far from the values expected geometrically; e.g. the Pb H bonds in PbH4 adopt sp1.8 hybridization although the geometry
is tetrahedral44 . This causes poor spatial orbital overlap and deviations from the ‘ideal’
geometries, a phenomenon called ‘hybridization defect’6 . Nevertheless, the s orbital contributions to the bonds are energetically more favorable than contributions from the larger
and more diffuse p orbitals. By using a high contribution of the s orbital in the hybridization of tetravalent compounds, the heavy elements also keep electron density close to
the nucleus as much as possible. Due to these large differences in the hybridization of


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