Synergic Back-Bonding

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. 

Correspondingly, the (empty) antibonding 2k molecular orbital favours the carbon  

In low oxidation states, transition metals possess filled or partly filled d shells. The Dewar-Chatt-Duncanson model envisages some of that electron density in (local) d (e.g. d., d y) orbitals being donated into the empty n orbitals of the carbon monoxide  

Donation of metal electron density into the molecular orbital of CO. An analogous process may be drawn in the yz plane. 

Accordingly, the CO moiety acquires negative charge. The consequent exigencies of the electroneutrality principle are then met by the CO group donating this charge back to the metal via its now expanded 7-donor orbital  

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Figure 6-13. Synergic back-bonding in a platinum alkene complex. In (a), the interaction of a (filled) platinum 5d orbital with the tf molecular orbital of the alkene is shown, whilst in (b), the interaction of a dsp hybrid orbital with the n molecular orbital of the alkene is shown. Note that the two interactions result in electron density moving in opposite directions.

E. J. Baerends and A. Rozendaal, in Quantum Chemistry The Challenge of Transition Metals and Coordination Chemistry, A. Veillard, Ed., Reidel, Dordrecht, 1986, pp. 159-177. Analysis of o-Bonding, it-(Back) Bonding and the Synergic Effect in Cr(CO)6. Comparison of Hartree-Fock and Xa Results for Metal-CO Bonding. 

In the side-on arrangement, the bonding is considered to arise from two interdependent components. In the first part, a overlap between the filled n orbital of N2 and a suitably directed vacant hybrid metal orbital forms a donor bond. In the second part, the M atom and N2 molecule are involved in two back-bonding interactions, one having it symmetry as shown in Fig. 15.1.7(a), and the other with S symmetry as shown in Fig. 15.1.7(b). These n and S-back bonds synergically reinforce the a bond. 

In many of their complexes PF3 and PPI13 (for example) resemble CO (p. 926) and this at one time encouraged the belief that their bonding capabilities were influenced not only by the factors (p. 198) which affect the stability of the a P M interaction which uses the lone-pair of elecU"ons on p and a vacant orbital on M, but also by the possibility of synergic n back-donation from a nonbonding d , pair of electrons on the metal into a vacant 3d , orbital on P. It is, however, not clear to what extent, if any, the a and n bonds reinforce each other, and more recent descriptions are based on an MO approach which uses all (cr and n) orbitals of appropriate symmeU"y on both the phosphine and the metal-containing moiety. To the extent that a and n bonding effects on the stability of metal-phosphorus bonds can be isolated from each otlier and from steric factors (see below) the accepted sequence of effects is as follows ... 

Such shifts can be rationalized in terms of the model of synergic a and ir bonding of the CO ligand in metal carbonyls (29) as was proposed by Blyholder (30). The formation of a strong surface bond involves considerable back donation from metal d... 

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