Organometallic compounds can be classified based on the nature of the metal-carbon bond:

1. $\sigma$-bonded complexes: The metal is bonded directly to the carbon atom via a single $\sigma$ bond. Example: $Fe(CH_3)_3$.
2. $\pi$-bonded complexes: The metal is bonded to the ligand via the ligand's delocalised $\pi$-electron cloud. Examples include Ferrocene $[Fe(\eta^5-C_5H_5)_2]$ and Zeise's salt $K[PtCl_3(\eta^2-C_2H_4)]$.
3. $\sigma$ and $\pi$ bonded complexes: Metal carbonyls exhibit both $\sigma$ and $\pi$ bonding character (synergic bonding). The CO ligand donates a lone pair of electrons to form a $\sigma$ bond with the metal, and the metal back-donates electron density from its filled d-orbitals into the empty $\pi^*$ antibonding orbital of CO to form a $\pi$ bond. Thus, the metal carbonyl complex $[Co(CO)_5NH_3]^{2+}$ is both $\sigma$ and $\pi$ bonded.