a
As the negative charge on metal carbonyl complex increases back pi bonding increases and hence the bond length of \(\mathrm{C}-O\) bond increases while the bond length of metal-carbon bond decreases. Hence. [Fe(CO), \(] ^{2-} {\text { has longest } C-O \text { bond length }}\) among the given complexes. The correct order of bond length of the given complexes is

\(\left[\mathrm{Mn}(\mathrm{CO})_{6}\right]^{+} < { \left[\mathrm{Ni}(\mathrm{CO})_{4}\right]}<\left[\mathrm{Co}(\mathrm{CO})_{4}\right]^{-}<\left[\mathrm{Fe}(\mathrm{CO})_{4}\right]^{2-}\)