Carbonyl stretching frequency as a probe of backbonding
= Carbonyl stretching frequency as a probe of backbonding
{title2=$\widetilde\nu_{\rm CO}\propto\sqrt{k_{\rm CO}/\mu}$}
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.