(મુક્ત $CO$ માં $C-O$ બંધલંબાઇ $1.128\,\mathop A\limits^o $ છે.
$\left[\mathrm{Mn}(\mathrm{CO})_{6}\right]^{+} < { \left[\mathrm{Ni}(\mathrm{CO})_{4}\right]}<\left[\mathrm{Co}(\mathrm{CO})_{4}\right]^{-}<\left[\mathrm{Fe}(\mathrm{CO})_{4}\right]^{2-}$
$(i)\, [CoCl_3 (NH_3)_3]$ $(ii)\, [Co(en)_3]Cl_3$
$(iii)\, [Co(C_2O_4)_2(NH_3)_2]^-$ $(iv)\, [CrCl_2(NH_3)_2(en)]^+$