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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.

A vibrational spectrum resolves transitions associated with molecular vibrations. In the harmonic approximation, a normal mode of angular frequency $\omega$ has levelsFor a diatomic molecule, $\omega \simeq \sqrt{k/\mu }$, where $k$ is the effective bond stiffness and $\mu$ the reduced mass. Replacing an atom by a heavier isotope therefore usually lowers the associated vibrational frequency without changing the basic electronic bonding.

Infrared spectroscopy detects a vibration when it couples to the electric dipole moment; Raman spectroscopy instead involves changing polarizability. Their selection rules can make complementary modes visible. Anharmonicity shifts frequencies and permits weaker transitions absent from the ideal harmonic picture.