Carbon monoxide forms stable metal carbonyls such as $\mathrm{Ni}(\mathrm{CO}){}_4$ mainly because $\mathrm{CO}$

1. Adonates a lone pair on oxygen to the metal and accepts no electron density back
2. Btransfers one electron to the metal to form an ionic $\mathrm{Ni}{}^{+}$ complex
3. Cis a pure $\pi$ donor through its filled $\pi$ bonding orbitals
4. Ddonates a lone pair on carbon to the metal and accepts metal $d$ electrons into its empty ${\pi }^{\ast }$ orbitals