Carbonyl stretching frequency as a probe of backbonding (${\tilde{\nu }}_{\mathrm{CO}}\propto \sqrt{k_{\mathrm{CO}}/\mu }$)

For a series of comparable metal carbonyls, greater metal-to-CO pi backbonding populates CO antibonding orbitals, weakens its internal bond and lowers its infrared stretching frequency. Coupled normal modes and changing geometry must be considered when comparing observed bands.