Bonding of Carbon Monoxide to Metals

Carbon monoxide has the valence bond structure shown as 

The bond order (B.O.) in the molecule is given in terms of the number of electrons in bonding orbitals (Nb) and the number in antibonding orbitals (Na) by 

The valence-bond approach to this multiple bonding can be shown in terms of the resonance structures  

In Fe(CO)5 the formal charge on iron is —5, and in Cr(CO)6 the formal charge on chromium is —6. We should expect that the back donation would be more extensive in either of these compounds than it is in the case of Ni(CO)4. Because the greater back donation results in a greater reduction in C-O bond order, the infrared spectra of these compounds should show this effect. The positions of the CO stretching bands for these compounds are as follows  

Although we have described terminal and bridging carbon monoxide, it is known to bond to metals in other ways. Some of the other types of linkages are shown as follows  

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