Question

In case of metal carbonyls, why does the bond order of CO decreases if the number of d electrons increases?

Answer 1

One possible explanation for the bond shortening after ionization is that the ionization leads to a shift of the COCO-electron-polarization (on ionization an electron is lost from the mostly CC-centered HOMO-σσ orbital and this leads to a formation of a positive partial charge on the CC atom). This strengthens the covalence of the COCO-bond and thus reduces the bond length (since the HOMO-σσ orbital is only slightly bonding there is not much bonding lost by taking away one of its electrons - the lost bonding is outweighed by the gain in covalency). You can think of this strenghtening of the bond covalence in the following way: Two atomic orbitals can interact better (form stronger bonds) if their energies are close. Without the positive partial charge on CC the AOs of OO lie energetically a lot below the AOs of CC (this is most pronounced for the ss AOs which form the HOMO-σ∗σ∗ orbital). But with the positive partial charge on CC the AOs of CC are shifted down in energy and thus their energies are closer to the related AOs of OO which leads to a stronger interaction when bonds are formed.