Most ligands have a nonbonding electron pair that may react as a donor to empty orbitals on the metal atom. In ligands known as π acids or π acceptors a donor-acceptor communication also happens in the reverse direction. If a ligand has empty orbitals of π type symmetry with respect to the bond axis these may react as acceptors for electrons in filled metal orbitals of the correct symmetry. That is known as back donation. The commonest and simplest π acid ligand is carbon monoxide CO. This reacts as a donor in the normal way, through the acquired lone-pair orbital centered on carbon (the 3σ MO). The π antibonding orbital may also interact with filled d orbitals to produce the π-acceptor interaction. The combination of π-acceptor and σ-donor interaction is sometimes defined as synergic, as the electron wants in opposite directions facilitate each other.

Evidence for the π-acceptor communication comes from various sources.

Fig. 1. Bonding in CO complexes showing (a) σ overlap of CO lone-pair with empty metal d orbital, and (b) overlap of CO π* with occupied metal d

orbital.