= 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.
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