Metal carbonyls contain synergic bond between metal atom and $\mathrm{C}\mathrm{O}$ molecules. It is $\mathrm{\sigma }$ donnor $(\mathrm{C}\to \mathrm{M})$ and $\mathrm{}\mathrm{\pi }$ acceptor $(\mathrm{M}\to \mathrm{C})$ bond (i.e. back-donation of electrons to antibonding molecular orbital from metal atom.)