(મુક્ત $CO$ માં $C-O$ બંધલંબાઇ $1.128\,\mathop A\limits^o $ છે.
$\left[\mathrm{Mn}(\mathrm{CO})_{6}\right]^{+} < { \left[\mathrm{Ni}(\mathrm{CO})_{4}\right]}<\left[\mathrm{Co}(\mathrm{CO})_{4}\right]^{-}<\left[\mathrm{Fe}(\mathrm{CO})_{4}\right]^{2-}$