The fields of hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) continue to attract the attention of researchers in the various disciplines connected to these fascinating problems that represent two of the key outstanding chemical challenges for the petroleum refining industry in view of their very strong environmental and commercial implications. One area that has flourished impressively over the last 15 years is the organometallic chemistry of thiophenes and other related sulfur-containing molecules. This has become a powerful method for modeling numerous surface species and reactions implicated in HDS schemes, and nowadays it represents an attractive complement to the standard procedures of surface chemistry and heterogeneous catalysis, for understanding the complex reaction mechanisms involved in this process. Similar developments have begun to appear in connection with HDN mechanisms, although in a much more modest scale and depth. Some years ago when, encouraged by Prof. B. R. James, this book was planned, several excellent reviews and monographs treating different aspects of HDS were already available including some on the subject of organometallic models. However, it seemed appropriate to try to summarize the most striking features of this chemistry in an updated and systematic way, and inasmuch as possible in connection with the common knowledge and beliefs of the mechanisms of heterogeneous HDS catalysis. Hopefully, this attempt to build some conceptual bridges between these two traditionally separated areas of chemistry has met with some success.

Computational methods have become an indispensible tool for elucidating the mechanism of organometallic reactions. This snapshot of state-of-the-art computational studies provides an overview of the vast field of computational organometallic chemistry. Authors from Asia, Europe and the US have been selected to contribute a chapter on their specialist areas. Topics addressed include: DFT studies on zirconium-mediated reactions, force field methods in organometallic chemistry, hydrogenation of -systems, oxidative functionalization of unactivated C-H bonds and olefins, the osmylation reaction, and cobalt carbonyl clusters. The breadth and depth of the contributions demonstrate not only the crucial role that computational methods play in the study of a wide range of organometallic reactions, but also attest the robust health of the field, which continues to benefit from, as well as inspire novel experimental studies.

While the organometallic chemistry of the d-block transition elements has been a flourishing field for the past 25 years, it has only been in the last several years that dramatic activity and progJ'ess has occurred in the area of lanthanide and actinide organometallic chemistry. The f. -element organometallic research effort has been truly multinational and multidisciplinary. In a large number of countries, sCientists have become increasingly interested in the synthesis, reactivity, spectroscopy, and the molecular and electronic structures of f-element organometallic compounds. The backgrounds of these scientists range from organic, inorganic, nuclear, and catalytic chemistry to chemical and nuclear physics. The motivations for the study of f-element organometallics have been equally varied. In the area of basic research, there has been a growing realization that the lanthanides and actinides represent two unique and, to a great extent, neglected families of elements in which many fascinating aspects of chemistry and bonding remain to be explored. On a more practical level, an increasing number of these elements play important roles in nuclear energy production and in industrial catalytic processes. It has become apparent that efficiency and safety in both areas could greatly benefit from increased knowledge. In the past there has been no suitable international forum available for bringing together researchers in the diverse areas of f-element organometallic science mentioned above.

Kyle A. Grice, Margaret L. Scheuermann and Karen I. Goldberg: Five-Coordinate Platinum(IV) Complexes.- Jay A. Labinger and John E. Bercaw: The Role of Higher Oxidation State Species in Platinum-Mediated C-H Bond Activation and Functionalization.- Joy M. Racowski and Melanie S. Sanford: Carbon-Heteroatom Bond-Forming Reductive Elimination from Palladium(IV) Complexes.- Helena C. Malinakova: Palladium(IV) Complexes as Intermediates in Catalytic and Stoichiometric Cascade Sequences Providing Complex Carbocycles and Heterocycles.- Allan J. Canty and Manab Sharma: h1-Alkynyl Chemistry for the Higher Oxidation States of Palladium and Platinum.- David C. Powers and Tobias Ritter: Palladium(III) in Synthesis and Catalysis.- Marc-Etienne Moret: Organometallic Platinum(II) and Palladium(II) Complexes as Donor Ligands for Lewis-Acidic d10 and s2 Centers.

Bismuth Catalysts in Aqueous Media, by Shu Kobayashi, Masaharu Ueno and Taku Kitanosono.- Pentavalent Organobismuth Reagents in Organic Synthesis: Alkylation, Alcohol Oxidation and Cationic Photopolymerization , by Yoshihiro Matano.- Environmentally Friendly Organic Synthesis Using Bismuth(III) Compounds, by Scott W. Krabbe and Ram S. Mohan.- Bismuth-Catalyzed Addition of Silyl Nucleophiles to Carbonyl Compounds and Imines, by Thierry Ollevier.- Bismuth Salts in Catalytic Alkylation Reactions, by Magnus Rueping and Boris J. Nachtsheim.- New Applications for Bismuth(III) Salts in Organic Synthesis: From Bulk Chemicals to Steroid and Terpene Chemistry, by J. A. R. Salvador, S. M. Silvestre, R. M. A. Pinto, R. C. Santos and C. Le Roux.- Cationic Bismuth-Catalyzed Hydroamination and Direct Substitution of the Hydroxy Group in Alcohols with Amides, by Shigeki Matsunaga and Masakatsu Shibasaki.- Transition-Metal Catalyzed C-C Bond Formation Using Organobismuth Compounds, by Shigeru Shimada and Maddali L. N. Rao.- Bismuth(III) Salts as Synthetic Tools in Organic Transformations, by J. S. Yadav, Aneesh Antony and Basi V. Subba Reddy.

This book serves as a concise guide to essential topics in Transition Metal Organometallic Chemistry for senior undergraduate and graduate students; it blends qualitative theoretical approach with experimental description of the facts. Its content emphasizes on the orbital description of M-L bonds; the electronic structures of the main types of organometallic complexes (ML2 to ML6); main types of organometallic reactions; organometallic compound synthesis, analytical characterization and the reactivity and lastly the applications of transition metals in homogeneous catalysis.

According to R.H. Crabtree, Metal Dihydrogen and sigma-Bond Complexes is described as 'the definitive account of twentieth-century work in the area of sigma complexation'. It covers not only Kubas' discovery of dihydrogen coordination and the study of its structure and general properties but also discusses both the theoretical beliefs and experimental results of bonding and activation of dihydrogen on metal centers and the coordination and activation of C-H, B-H, X-H, and X-Y bonds, giving an overview of 'one of the hottest areas in chemistry'.

Juan I. Padron and Victor S. Martin: Catalysis by means of Fe-based Lewis acids; Hiroshi Nakazawa*, Masumi Itazaki: Fe-H Complexes in Catalysis; Kristin Schroder, Kathrin Junge, Bianca Bitterlich, and Matthias Beller: Fe-catalyzed Oxidation Reactions of Olefins, Alkanes and Alcohols: Involvement of Oxo- and Peroxo Complexes; Chi-Ming Che, Cong-Ying Zhou, Ella Lai-Ming Wong: Catalysis by Fe=X Complexes (X=NR, CR2); Rene Peters, Daniel F. Fischer and Sascha Jautze: Ferrocene and Half Sandwich Complexes as Catalysts with Iron Participation; Markus Jegelka, Bernd Plietker: Catalysis by Means of Complex Ferrates."

The present book is based on the work of M.N.Bochkarev, G.S.Kalinina, L.N. zakharov and S.Ya.Khorshev. The Russian edition of that book appeared under the same title in 1989 and covered literature data up to the middle of 1986. Since that time the number of publications on this subject increased significantly. In this volume we include all the data published up to the end of 1990, as well as some of the most important relevant articles of 1991. Therefore, this book should be considered as a new book, devoted to the same problems, rather than as just a translation of the mentioned issue. This book deals with compounds of scandium, yttrium, lanthanum and lanthanoids containing direct metal-carbon bond, Le. with the real organometallic complexes of these metals. Besides, the volume includes the rare earth complexes, in which organic ligand is bonded to the metal atom via the atom of another element of the Periodic Table. In other words, the book includes all classes of rare earth organoderivatives. Carboxilates, fl-diketonates and related chelates are the exceptions, because their properties are closer to inorganic compounds and they were fully described elsewhere. It should be noted, that "rare earth elements," "rare earth metals," "lanthanoids" and related terms are used in this book for indicating scandium, yttrium, lanthanum and the following 14 elements of the Periodic Table.

The chemistry of transition metal carbyne complexes has become a highly attractive field during the past twenty years. In recent years its application to aspects of catalysis and metathesis has gained considerable interest from inorganic as well as organic chemists. In addition, organic synthesis by means of metal carbon multiple bond reagents offers the most sophisticated technology currently available. In consideration of these developments some of Professor E. O. Fischer's former coworkers and colleagues felt obliged to orga nize this NATO Advanced Research Workshop on Transition Metal Carbyne Complexes in the Bavarian Alps. They have been encouraged by the fact that most of the distinguished scientists in the field of metal-carbon multiple bond chemistry had finally agreed to participate and to present stimulating lectures. The organizers of the workshop are deeply grateful to the Scientific Affairs Division of the NATO for the generous financial support of the meeting in Wildbad Kreuth and for the preparation of this book. They also feel indebted to acknowledge the generous support from Wacker-Chemie, BASF, Peroxid-Chemie, Hoechst and Bayer. Finally they thank the staff of the Hanns-Seidel-Stiftung in Wildbad Kreuth for providing a pleasant and stimu lating atmosphere during the meeting."

Contents: Gerard Jaouen, Nils Metzler-Nolte : Introduction ; Stephane GIBAUD and Gerard JAOUEN: Arsenic - based drugs: from Fowler's solution to modern anticancer chemotherapy; Ana M. Pizarro, Abraha Habtemariam and Peter J. Sadler : Activation Mechanisms for Organometallic Anticancer Complexes; Angela Casini, Christian G. Hartinger, Alexey A. Nazarov, Paul J. Dyson : Organometallic antitumour agents with alternative modes of action; Elizabeth A. Hillard, Anne Vessieres, Gerard Jaouen : Ferrocene functionalized endocrine modulators for the treatment of cancer; Megan Hogan and Matthias Tacke : Titanocenes - Cytotoxic and Anti-Angiogenic Chemotherapy Against Advanced Renal-Cell Cancer; Seann P. Mulcahy and Eric Meggers : Organometallics as Structural Scaffolds for Enzyme Inhibitor Design; Christophe Biot and Daniel Dive : Bioorganometallic Chemistry and Malaria; Nils Metzler-Nolte : Biomedical applications of organometal-peptide conjugates; Roger Alberto : Organometallic Radiopharmaceuticals; Brian E. Mann : Carbon Monoxide - an essential signaling molecule.

The scope of this paper is to recall fundamental notions of the molecular spectroscopy and dynamics, necessary for discussion of photophysical and photochemical processes in condensed phases. We will thus treat in a more detailed way the specific features which are important for molecular systems strongly interacting with their environment. Other aspects such as the time evolution of isolated molecules, single-level excitation and state-to-state chemistry, important for the gas-phase photophysics are omitted. We start (Sec.2) with a brief description of radiative processes (light absorption and emission) in molecules. In the quantum-mechanical treatment of this problem, the appropriate basis is that of so-called zero-order states, corresponding to the traditional scheme of electronic states (singlets, doublets, triplets etc.) and vibrational levels belonging to each state. The important point will be deduction of selection rules for most radiative transitions. At this stage all molecular states are considered as stationary states. In order to treat the breakdown of simple selection rules and non-radiative transitions between individual molecular states, it is necessary to take into account the mechanisms coupling the zero-order states (Sec.3). We will first focus on intramolecular coupling effects and then discuss the solvent effects on intramolecular relaxation processes. The problem of the non-radiative transfer of the electronic energy between different molecules - closely related to that of the energy dissipation within a single molecule will be treated in Sec.4.

The analogy between the chemistry of molecular transition metal clusters and the processes of chemisorption and catalysis at metal surfaces (the Cluster Surface analogy) has for a number of years provided an interplay between experimental and theoretical inorganic and physical chemists. This collaborative approach has born fruit in the use of well defined modes of metal-ligand bonding in discrete molecular clusters, models for metal-ligand binding on surfaces. Some of the key topics discussed in The Synergy between Dynamics and Reactivity at Clusters and Surfaces are: (1) Mechanisms of the fluxional behaviour in clusters in the liquid phase and the connections with diffusion processes on extended surfaces. The role of metal-metal bond breaking in diffusion. (2) Analogies in the structure of chemisorbed species and related ligands on metallic clusters. (3) Analogies between benzene surface chemistry on extended metal surfaces and on metal surfaces in molecular cluster compounds with particular reference to structural distortions. (4) The role of mobile precursors for dissociation of chemisorption on extended metals and on clusters. Are there analogies in the ligand attachment during cluster compound synthesis? (5) The role of defect sites on metal surfaces in catalyzing chemical reactions and the connection to the special bonding properties of sites on metal clusters having lowest metal-metal coordination. (6) The size of metal clusters needed to mimic surface phenomena on bulk metal surfaces. Different sites needed for different phenomena.

In the last 15 years aqueous organometallic chemistry and catalysis has emerged from being a laboratory curiosity to become an established field of research. Topics reviewed here include mechanistic studies on the effect of water on catalyzed reactions, the preparation of water soluble phosphines as ligands for catalysis, metal catalyzed organic reactions in water (hydrogenation, hydroformylation, carbonylation, olefin metathesis, hydrophosphination, etc.), chiral ligands and enantioselective catalysis, organometallic radical photochemistry in aqueous solutions, bioorganometallic chemistry, organometallic reactions of biopolymers, and catalytic modification of biomembranes. The summary of recent results is supplemented by an assessment of probable future research trends. Audience: Researchers in both academia and industry, as well as graduate students of homogeneous catalysis.

The activation of dioxygen by metal ions has both synthetic potential and biological relevance. Dioxygen is the cleanest oxidant for use in emission-free technologies to minimize pollution of the environment. The book gives a survey of those catalyst systems based on metal complexes which have been discovered and studied in the last decade. They activate molecular oxygen and effect the oxidation of various organic compounds under mild conditions. Much of the recent progress is due to a search for biomimetic catalysts that would duplicate the action of metalloenzymes. Mechanistic aspects are emphasized throughout the book. An introductonary chapter reviews the chemistry of transition metal dioxygen complexes, which are usually the active intermediates in the catalytic reactions discussed. Separate chapters are devoted to oxidation of saturated, unsaturated and aromatic hydrocarbons, phenols, catechols, oxo-compounds, phosphorus, sulfur and nitrogen compounds.

Masakatsu Shibasaki, Motomu Kanai, Shigeki Matsunaga, and Naoya Kumagai: Multimetallic Multifunctional Catalysts for Asymmetric Reactions.- Takao Ikariya: Bifunctional transition metal-based molecular catalysts for asymmetric syntheses.- Chidambaram Gunanathan and David Milstein: Bond Activation by Metal-Ligand Cooperation: Design of Green Catalytic Reactions Based on Aromatization-Dearomatization of Pincer Complexes.- Madeleine C. Warner, Charles P. Casey, and Jan-E. Backvall: Shvo s Catalyst in Hydrogen Transfer Reactions.- Noritaka Mizuno, Keigo Kamata, and Kazuya Yamaguchi: Liquid-Phase Selective Oxidation by Multimetallic Active Sites of Polyoxometalate-Based Molecular Catalysts.- Pingfan Li and Hisashi Yamamoto: Bifunctional Acid Catalysts for Organic Synthesis.- Jun-ichi Ito, Hisao Nishiyama: Bifunctional Phebox Complexes for Asymmetric Catalysis."

Structure, Bonding, and Reactivity of Reactant Complexes and Key Intermediates, by Elena Soriano and Jose Marco-Contelles.-

Cycloisomerization of 1, "n"-Enynes Via Carbophilic Activation, by Patrick Yves Toullec and Veronique Michelet.-

DFT-Based Mechanistic Insights into Noble Metal-Catalyzed Rearrangement of Propargylic Derivatives: Chirality Transfer Processes, by Olalla Nieto Faza and Angel R. de Lera.-

"N"-Heterocyclic Carbene Complexes of Au, Pd, and Pt as Effective Catalysts in Organic Synthesis, by Andrea Correa, Steven P. Nolan and Luigi Cavallo.-

Activation of Allenes by Gold Complexes: A Theoretical Standpoint, by Max Malacria, Louis Fensterbank and Vincent Gandon.-

Heterocyclization of Allenes Catalyzed by Late Transition Metals: Mechanisms and Regioselectivity, by Benito Alcaide, Pedro Almendros, Teresa Martinez del Campo, Elena Soriano and Jose Marco-Contelles.-

Gold-Catalyzed Cycloadditions Involving Allenes: Mechanistic Insights from Theoretical Studies, by Sergi Montserrat, Gregori Ujaque, Fernando Lopez, Jose L. Mascarenas and Agusti Lledos.-

"

The serious study of the reaction mechanisms of transition metal com plexes began some five decades ago. Work was initiated in the United States and Great Britain; the pioneers ofthat era were, inalphabetical order, F. Basolo, R. E. Connick, 1. O. Edwards, C. S. Garner, G. P.Haight, W. C. E. Higgision, E.1. King, R. G. Pearson, H. Taube, M.1. Tobe, and R. G. Wilkins.A larger community of research scientists then entered the field, many of them stu dents ofthose just mentioned. Interest spread elsewhere as well, principally to Asia, Canada, and Europe. Before long, the results ofindividual studies were being consolidated into models, many of which traced their origins to the better-established field of mechanistic organic chemistry. For a time this sufficed, but major revisions and new assignments of mechanism became necessary for both ligand sub stitution and oxidation-reduction reactions. Mechanistic inorganic chemistry thus took on a shape of its own. This process has brought us to the present time. Interests have expanded both to include new and more complex species (e.g., metalloproteins) and a wealth of new experimental techniques that have developed mechanisms in ever-finer detail. This is the story the author tells, and in so doing he weaves in the identities of the investigators with the story he has to tell. This makes an enjoyable as well as informative reading."

Alexander L. Reznichenko and Kai C. Hultzsch: Catalytic -Bond Metathesis Zhichao Zhang, Dongmei Cui, Baoli Wang, Bo Liu, Yi Yang: Polymerization of 1,3-Conjugated Dienes with Lanthanide Precursors Frank T. Edelmann: Homogeneous Catalysis using Lanthanide Amidinates and Guanidinates Tianshu Li, Jelena Jenter, Peter W. Roesky: Rare Earth Metal Post-metallocene Catalysts with Chelating Amido Ligands

Zeolites, with their crystalline microporous structures, are cordial hosts to a wide variety of guests. However, it was the abrupt and unexpected departure of one of these guests (water) from a host (stilbite) on heating which led Cronstedt, in 1756, to coin the term "zeolite" (from the Greek meaning "boiling stone") to describe this material. Since that time, approximately 40 different naturally-occurring zeolites have been discovered on earth. Recent studies of meteorite compositions have shown that these guest-host materials (e. g. , sodalite) occur in other parts of the universe as well. However, it wasn't until the twentieth century that synthetic routes to zeolites and other non-aluminosilicate molecular sieves were discovered. In addition, with the development of X-ray diffraction and the various spectroscopies, better understanding of the nature of the cavities, cages, and channels of these materials has led to the industrial exploitation of their guest-host properties. The world of zeolites has now expanded into a greater than 2 billion pound per year business, with major applications in detergent formulations, catalysis, and as adsorbents and desiccants. Their economic impact is difficult to determine; however, the improvement in gasoline yields alone (from catalytic cracking) must account for hundreds ofbillions ofdollars in increased GDP. In this volume, we have brought together a sampling of recent developments in various areas of guest-host or inclusion chemistry in zeolites.

Homogeneous catalysis plays an important role both in the laboratory and in the industry. Successful applications in industry involve new polymerisation processes with complexes of zirconium and related metals, new carbonylation processes employing palladium and rhodium, ring opening polymerisations, and new enantioselective isomerisation catalysts as in the preparation of menthol. Also in the synthesis of organic compounds in the laboratory highly selective homogeneous catalysts represent an irreplaceable part of the toolbox of the synthetic chemist. Examples of such reactions are cross-coupling (Ni, Pd), nucleophilic substitution of allylpalladium complexes, Heck reactions (Pd), asymmetric epoxidation, Wacker type reactions (Pd), asymmetric hydrogenations (Rh, Ru), reactions of chromium complexes, enantioselective reactions with Lewis acids, reactions with the McMurry reagent, etc. There is hardly any multistep organic synthesis that does not involve one of these metal catalysed reactions. Most of these catalysts have been developed by empiricism. The metal catalysed processes consist of a series of elementary steps which often have been studied in isolation in organometallic chemistry. The knowl- edge of such elementary steps - effect ofligands, anions, coordination number, valence states - has greatly contributed to the development of improved cata- lysts for the reactions mentioned above. In addition to the empirical approach theoretical methods have given support and guidance to the development of improved processes. Often the key steps of a cycle escape from a direct ob- servation and then theoretical contributions are even more wanted.

Arvind Kumar, Shih-Sheng Sun, and Alistair J. Lees: Photophysics and Photochemistry of Organometallic Rhenium Diimine Complexes; Conor Long: Photophysics of CO Loss from Simple Metal Carbonyl Complexes; Antonin Vlcek Jr: Ultrafast Excited-State Processes in Re(I) Carbonyl-Diimine Complexes: From Excitation to Photochemistry; Kenneth Kam-Wing Lo: Exploitation of Luminescent Organometallic Rhenium(I) and Iridium(III) Complexes in Biological Studies; Maria L. Muro, Aaron A. Rachford, Xianghuai Wang, and Felix N. Castellano: Platinum II Acetylide Photophysics; Andreas F. Rausch, Herbert H. H. Homeier, and Hartmut Yersin: Organometallic Pt(II) and Ir(III) Triplet Emitters for OLED Applications and the Role of Spin Orbit Coupling: A Study Based on High-Resolution Optical Spectroscopy."

Contents: Yves Canac and Remi Chauvin: Neutral eta1-carbon ligands: beyond carbon monoxide; Esteban P. Urriolabeitia: Ylide Ligands; Wolfgang Petz and Gernot Frenking: Carbodiphosphoranes and related ligands; Mareike C. Jahnke and F. Ekkehardt Hahn: Chemistry of N-Heterocyclic Carbene Ligands; Tsuyoshi Kato, Eddy Maerten, Antoine Baceiredo: Non-NHCs stable singlet carbene ligands; Victorio Cadierno, Sergio E. Garcia-Garrido: All-Carbon-Substituted Allenylidene and Related Cumulenylidene Ligands; Victorio Cadierno, Sergio E. Garcia-Garrido: Heteroatom-Conjugated Allenylidene and Related Cumulenylidene Ligands."

217 2. COPOLYMERIZATION OF PROPENE OR HIGHER I-ALKENES WITH 218 CARBON MONOXIDE 2. 1. Ligands and polymerization conditions 218 2. 2. Spiroketal formation 221 2. 3. Enantioselectivity 222 2. 4. Higher I-Alkenes 226 3. COPOLYMERIZATION OF STYRENE OR ITS DERIVATIVES WITH 226 CARBON MONOXIDE 4. COPOLYMERIZATION OF OTHER OLEANS WITH CARBON MONOXIDE 230 5. ASYMMETRIC TERPOLYMERIZATION OF MORE THAN Two KINDS OF 232 OLEFINS WITH CARBON MONOXIDE 6. POLYKETONE CONFORMATION 233 7. CONCLUSION 234 Chapter 8. Chain Propagation Mechanisms 237 Ayusman Sen 1. INTRODUCTION 237 2. PALLADIUM (II) BASED SYSTEMS 238 3. NICKEL (II) BASED SYSTEMS 256 4. RHODIUM (I) BASED SYSTEMS 257 5. CONCLUSION 261 Chapter 9. Theoretical Studies on Copolymerization of Polar Monomers 265 Peter Margl, Artur Michalak, and Tom Ziegler 1. INTRODUCTION 265 2. COPOLYMERIZATION OF CARBON MONOXIDE WITH ETHYLENE 267 2. 1. Experimental and calculated rates for the insertion processes for 267 copolymerization catalysed by Pd(II) systems. 2. 2. A more detailed look at the productive and unproductive cycles 270 in copolymerization catalysed by Pd(II) complexes. 2. 2. 1. The productive cycle 270 2. 2. 2. C2H4 misinsertion into an ethylene terminated polyketone 275 chain 2. 3. Experimental and calculated rates for the insertion processes for 277 alternating copolymerization catalyzed by Ni(II) systems 3. COPOLYMERIZATION OF OLEFINS WITH POLAR MONOMERS OTHER 280 THAN CO 3. 1. Preferred binding mode of oxygen containing monomers 282 3. 2. Preferred binding mode of nitrogen containing monomers 285 3. 3.

A very large number of organo derivatives is formed by the Group IV ele ments silicon, germanium, tin, and lead. In comparing the general properties of these elements, Table 1. 1 shows that the first ionization energies decrease (though not in a regular way) with increase in size and atomic number, con sistent with the general increase in metallic character of the elements. Electro negativity values (which have been the subject of considerable controversy) show no clear trend. Although purely inorganic compounds of tin(n) and leaden) are well known, almost all organo Group IV derivatives show an oxidation state of IV. Bonds to carbon become weaker on passing from silicon to lead, as do the element-element bonds themselves. With any particular element M (M = Si, Ge, Sn, or Pb), there is a small decrease in bond energy in the order: M-Ph > M - Me > M - Et. Although accurate data for organo derivatives are lacking, strengths of bonds to other elements probably decrease in the order: M-F> M-O > M-CI > M-H ~ M-N ~ M-S ~ M-Br > M-I, while for a particular element X, the order is: Si-X > Ge-X > Sn-X > Pb-X. It is therefore understandable that reactions leading to Si-F, Si-O, or Si-CI bonds are especially favoured in a thermodynamic sense.