The bonding in metal-tricarbonyl groups

The three sets of orbitals, namely the a-, jr-tangential- and n-radial-orbitals, are not interchanged by any of the symmetry operations of the group Csv. The three orbitals in each set combine to give the molecular 

The three sets of carbonyl molecular orbitals all contain M.O.S of E symmetry and the a and jr-radial-orbitals also form M.O.s of Ai symmetry. It follows that the simple picture of M-CO bonding, namely donation to the metal by the CO r-orbital and back-donation from the metal to the carbonyl 7i-orbitals, breaks down since the metal orbitals of both At and E symmetry take part in both donor and acceptor interactions. 

s formed by the jr-radial- or 7i-tangential-orbitals are not necessarily equally involved in bonding to the metal and one consequence of this would be that the M-CO system would be non-linear, which is frequently found [129u]. The deviation from non-linearity would obviously depend on the effects of other ligands attached to the metal. 

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