Carbon monoxide Ligand properties

A lone pair of electrons are available on both carbon and oxygen atoms of a carbon monoxide ligand. However, as the carbon atoms donate electrons to the metal, these complexes are named carbonyls. A variety of such complexes, such as mononuclear, polynuclear, homoleptic and mixed ligands, are known. These compounds are widely studied due to their abUity to release carbon monoxide [3], their industrial importance, their catalytic properties [4] and their structural interest [5]. Carbon monoxide is one of the most important TC-acceptor ligands. Because of its TC-acidity, carbon monoxide can stabilize the zero formal oxidation state of metals in carbonyl complexes. 

As was mentioned earlier (p 189), an important property of transition metal a-alkyls is their ability to rmdergo insertion reactions, especially with carbon monoxide, when acyl complexes are formed. Frequently such carbonylation reactions are reversible. Studies on the decarbonylation of acetylmanganese pentacarbonyl labelled with in the acetyl-CO group indicate that this carbonyl group is retained in the molecule as a carbon monoxide ligand ... 

The lobes of electron density outside the C-O vector thus offer cr-donor lone-pair character. Surprisingly, carbon monoxide does not form particularly stable complexes with BF3 or with main group metals such as potassium or magnesium. Yet transition-metal complexes with carbon monoxide are known by the thousand. In all cases, the CO ligands are bound to the metal through the carbon atom and the complexes are called carbonyls. Furthermore, the metals occur most usually in low formal oxidation states. Dewar, Chatt and Duncanson have described a bonding scheme for the metal - CO interaction that successfully accounts for the formation and properties of these transition-metal carbonyls. 

It should be recognized that the stability of cobalt complexes under carbon monoxide can be enhanced by the addition of ligands, as is the case for phosphine-modified cobalt hydroformylation catalysts (57, 58). The stability will also probably depend on properties of the solvent employed. Nevertheless, the plot shown in Fig. 4 appears to be quite useful for assessing long-term cobalt stability under H2/CO in the absence of strongly coordinating solvents or ligands. 

Fig. 11 0 Competition by ligands for Ihe ir bonding d orbilul of a central metal atom. Relative overtop is symbolized by the shaded areas, (a) Equal aod strong tr bonds resulting from equal and good overlap of Ihe two carbon monoxide sr orbitals with the meial J orbital (b) Superior overlap of carbon monoxide t orbital wilh polarized metal d orbiial compared lo poorer overlap between ligand <1 and metal d orbitals. Polarization (mixing of higher energy wave functions) occurs so as to maximize total overlap Recall that the overlap integral includes both spatial and intensive properties Ihe represemation above is a graphic simplification.

Matrix isolation methods of synthesis have also been used to prepare and study coordination compounds. These involve the vaporization of a metal and a potential ligand, which are then rapidly carried in a stream of inert gas to a very cold surface, where the compound which has been formed is quickly trapped in the solid matrix. It is possible to determine the type of bonding, the structure and the thermodynamic properties of the compounds formed. Only small ligand molecules have been used thus far carbon monoxide, nitric oxide, nitrogen and oxygen, for example, but molecules of great interest have been formed. Some such are [Pd(C2H4)], [Pd(N2)3], [Ni(N2)202], [Ni(N2)4] and [Ni(CO)(N2)3].41... 

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