The correct option (d) 1.118Å
Explanation:
The metal (Fe) makes the back bonding to CO. The metal to ligand bonding creates a synergic effect which strengthens the CO and the metal (Fe) bond. Which results in the contraction of CO bond length (Synergic means working together towards a common end. This is a bond formed by a ligand to metal ion which has a σ -bond from ligand to metal ion and the back donation of electron density from the metal to the π- orbitals on the ligand. Thus enhancing the acceptor strength of the p-orbital. Hence, upto a point the effect of σ-bond formation strengthens the π -bonding or vice-versa. Due to this synergic effect, bond order decreases).
