Ligand properties monoxide

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... 

It is known from a variety of physical and chemical evidence that CO is a poor a donor and a good 7t acceptor, but the opposite is true of CN, while CNH can be expected to have intermediate properties. In a formal sense, carbon monoxide may be derived from the ligand CNH by coalescence of the nitrogen and hydrogen nuclei. The general observation, made in molecular spectroscopy, is that the electronic structure of a diatomic hydride resembles the combined atom more closely than the separated atoms. Thus, it is reasonable that the electronic structure of CNH will resemble CO more closely than CN-. This is also borne out by a semi-empirical self-consistent molecular orbital calculation, the results of which are summarized in Fig. 7. 

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