Metal carbonyl

A metal carbonyl is a coordination complex containing carbon monoxide ligands. The ligand normally binds through carbon, donating an electron pair while accepting metal electron density into an antibonding orbital through pi backbonding.

These complementary interactions explain a useful spectroscopic test. Stronger backbonding tends to weaken the internal carbon–oxygen bond and lower its stretching frequency relative to weakly backbonded CO. A vibrational spectrum therefore probes electronic bonding, although terminal versus bridging coordination and the other ligands also affect the observed bands. Simple electron counting treats a neutral terminal CO as a two-electron donor, but that convention is not a complete description of bond strength.