A) [Fe(CO)5] & [Ni(CO)4] complexes have 18-electrons in their valence shell.B) Carbonyl complexes are predominantly low spin complexes due to strong ligand field.C) As electron density increases on metals (with lowering oxidation state on metals), the extent of synergic bonding increases. Hence M–C bond strength increases.D) While positive charge on metals increases and the extent of synergic bond decreases and hence C–O bond becomes stronger.