Organometallic Chemistry

Fundamentals and Applications

Ionel Haiduc

• Editore: De Gruyter
• Anno: 2022
• ISBN:
  9783110695441
• Edizione: 2
• Argomento: Physical Sciences
• Categoria: Inorganic Chemistry

This book provides the reader with a comprehensive introduction to the topic of organometallic chemistry. With an easy to follow structure covering both nontransition metals and transition metals as well as the applications of organometallic reagents in organic synthesis, this book is a must-have for the organometallic chemist.

Part I: General

1 The scope of organometallic chemistry

Organometallic chemistry is the discipline dealing with compounds containing at least one direct metal–carbon bond. It should be added that organometallic chemistry deals with compounds in which an organic group is attached to an atom which is less electronegative than carbon (electronegativity X = 2.50). On this basis, the organic derivatives of some non-metals (with the electronegativities shown in brackets), namely boron (X = 2.01), silicon (X = 1.74) and arsenic (X = 2.20) are traditionally included in organometallic chemistry, although these elements are not metals (but often described as metalloids). In this volume, the organic derivatives of B, Si, Ge and As are not included, assuming that their metallic character is not predominant.

All elements, except the noble gases (other than xenon), form compounds with element–carbon bonds. Therefore, organometallic chemistry embraces the organic derivatives of the alkali and alkaline earth metals, the non-transition metals (Main groups 13–15), the transition metals (d-block elements, plus lanthanides and actinides) and some nonmetals (or metalloids) such as boron, silicon, antimony and tellurium.

It should also be mentioned that several classes of compounds which contain a metal and carbon in their composition, are not described as organometallic if a direct metal–carbon bond is absent. Thus, compounds such as metal alkoxides (with M^_OR bonds), metal amides (with M–N bonds), chelate complexes (e.g., acetylacetonates) or the metal salts of carboxylic acids, are not considered organometallic. Often, such compounds are described as metal organic.

Perhaps a classification as described may seem somewhat arbitrary but it is practical and generally accepted.

2 Organometallic molecules: the nature of metal–carbon bonds

The stability and reactivity of various organometallic compounds are very different. Some are very sensitive and react spontaneously with oxygen and water or are thermally unstable, whereas others are perfectly stable and can be handled in open atmosphere at room temperature without any special precautions. The chemical properties of organometallic compounds are determined by the nature of the metal–carbon bonds, and they can differ very much between various metals.

The classical bond types, covalent and ionic, are present in many organometallic compounds, but there are some metal–carbon interactions discovered and present only in the metal–carbon compounds.

2.1 Sigma-covalent (bicentric bielectronic) metal–carbon bonds

These are the classical covalent bonds formed by pairing of two electrons of opposite spin and are possible for all elements.

These are typical for all main group (nontransition) elements but also occur in transition metal derivatives. Because of the electronegativity differences between carbon and metals, the covalent metal–carbon bonds are polar M^δ+–C^δ– covalent bonds, and the degree of polarity (percent of ionic character) depends on the electronegativity difference. The stability of polar covalent bonds is influenced by the nature of organic substituents. Electron-attracting substituents in the organic group (e.g., fluorine) increase the stability of the M–C bonds. This is reflected in the fact that the M–CF₃ and M–C₆F₅ derivatives are significantly thermally more stable than the nonfluorinated analogues, especially in the case of transition metals.

The stability of σ-covalent organometallic compounds is determined by thermodynamic and kinetic factors.

The main group elements form homoleptic compounds of MR_n type, where n is the typical valence of the metal, which are in general thermally stable. The transition metals show less tendency to form stable homoleptic compounds; their low stability is of kinetic origin and is due to incomplete occupation of the d orbitals. Stability is gained by adding π-acceptor ligands like CO, PR₃ and π-C₅H₅, which form additional dative bonds to increase the kinetic stability. Thus, Ti(CH₃)₄ is unstable at room temperature but the cyclopentadienyl derivative (π-C...