IIT JEE Advanced 2006 Paper 1

In the complex Fe(CO)5, the bond formed by the ligand (CO) with the metal (Fe) is a sigma (σ) and metal forms a back bond with the ligand by donation of electron density from π* orbitals. The effect of formation of both the bonds is synergic and strengthens the Fe-CO bond, thus leading slight decrease in bond order of CO bond. Hence, the bond length of CO is slightly more in the complex.