Bonding in organometallic -complexes

Figure 2-61. a) The bonding in organometallic complexes (e.g., ferrocene) cannot be expressed adequately by a connection table, b) A new representation has to account for all the valence electrons of Iron,... 

The readers will be aware of the basic principles of bonding in organometallic complexes, especially the bonding of alkenes and carbon monoxide, and this will not be dealt with here. We will say a few words about ligands containing phosphorus as the donor element. 

Insertion of unsaturated molecules into a transition metal-silyl bond has been suggested for the catalytic reactions related to hydrosilylation and silylcarbonylation. However, there is little direct evidence supporting such a process for unsaturated molecules to insert into a metal-silyl bond in organometallic complexes. " Thus, the fact that 108 is readily derived from 11 and 13 demonstrates the participation of this process in the catalytic cycle of silylformylation. 

The first section summarizes simply the essential features of the different types of molecular orbital calculations currently used to solve theoretical problems in organometallic chemistry. A critical comparison of these calculations is also given. The second section discusses the bonding in organometallic complexes and draws on recent computational results and develops the chemical and structural implications of bonding models based on perturbation theory arguments. 

D. Braga, etal., J. Am. Chem. Soc. 1995,117,5156 (C—H OCM H-bonding in first row metal carbonyls) Organometallics 1996, 15, 2692 (M—H O H-bonding in organometallic complexes and clusters Data from Cambridge Crystallographic Data Centre). 

A huge number of different synthetic approaches to generate mgm/tm bonds in organometallic complexes exist. The most widely applicable can be categorized as follows ... 

The orbital interaction scheme in which the attractive contribution of the adsorbate surface bond is estimated from the donative and respective back-donative interactions is called the Blyholder model. It is the analogous to the Chatt-Dewar model which is used to describe chemical bonds in organometallic complexes. 

Group 4.- As in the past three years, there appear to be no reports of direct metal-metal bonds in organometallic complexes of this group. 

Examples of carbyne-metal bonds in organometallic complexes, the adequate model being the framed one, a robust cluster... 

C. A molecular orbital description of the bonding in organometallic complexes. 

The number of complexes for which the 18-electron rule is an overriding bonding condition is very large. It is an essential tool for interpreting the bonding in organometallic compounds and it will be considered many times in the chapters to follow. 

Use of the r]n- prefix in organometallic complexes is explained in Chapter 18. The superscript n gives the number of atoms (usually carbon atoms) in a ligand that are directly bonded to the metal center for example, rf-cyclopentadienide means that all five carbons in a CsHs- ligand are bonded to the metal atom. 

Braga, D., Grepioni, F., Sabatino, P., Desiraju, G. R., Hydrogen-bonding in organometallic crystals. 1. from carboxylic-acids and alcohols to carbonyl-complexes. Organometallics 1994, 13, 3532-3543. 

We have learned that the atomic orbitals on transition metal surfaces or in organometallic complexes are rehybridized to a significant extent. This changes the nature of the chemical bond considerably from that in the free atom. We will illustrate this by comparing dissociation of the H2 molecule by a Ni atom and the Ni(PH3)2 complex [15-18]. 

In studies on the absorption-edge fine structure in organometallic complexes, Pauling s valence bond theory may be used in explaining the fine structure. 

In organometallic complexes of alkenes, where a transition metal uses an empty d orbital to coordinate to the n bond of the alkene, the vinyl carbons move upfield to the range of 8 7-110, varying widely and depending on the exact structure of the complex.5 The reasons for this upfield shift include both shielding by the electron-rich electropositive metal, and a change in hybridization of the vinyl carbons toward less s character (i.e., more sp3 like). 

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