# How is backbonding treated in the theory of covalent bonding?

Backbonding is usually invoked in coordination chemistry, where an electron donor, a ligand, binds to, coordinates to a Lewis-acidic metal centre by donating electrons. If the metal is already in a low oxidation state, further coordination MAY not occur, unless there is a mechanism by which the metal can back-donate electron density to the ligand. Ligands such as carbon monoxide are known as $\pi - \text{acids}$, given their ability to accept electron density from a metal centre......
For complexes such as $\left[C r C {O}_{6}\right]$ the $C - O$ stretching frequency observed in the IR spectrum is MARKEDLY reduced because the zerovalent metal DONATES electron density to the $\pi \text{*}$, antibonding orbitals of carbon monoxide, which are UNFILLED in ""^(-):C-=O:^+. This interaction is ANTIBONDING with respect to the $C - O$ bond, and thus its stretching frequency is reduced in the IR spectrum with respect to free $C O$; but it IS BONDING with respect to $C r - C$ as it places more electron density between the metal and the binding carbon.