 The energies of the νCO band for the metal carbonyls correlates with the strength of the carbon-oxygen bond, and inversely correlated with the strength of the π-backbonding between the metal and the carbon.The π basicity of the metal center depends on a lot of factors; in the isoelectronic series (Ti to Fe) at the bottom of this section, the hexacarbonyls show decreasing π-backbonding as one increases (makes more positive) the charge on the metal.π-Basic ligands increase π-electron density at the metal, and improved backbonding reduces νCO.