Question

How is backbonding treated in the theory of covalent bonding?

Answer 1

All covalent bonding is marked by the sharing of electrons between two atoms.

Explanation:

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-"acids"`, given their ability to accept electron density from a metal centre......

For complexes such as `[CrCO_6]` the `C-O` stretching frequency observed in the IR spectrum is MARKEDLY reduced because the zerovalent metal DONATES electron density to the `pi"*"`, 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 `CO`; but it IS BONDING with respect to `Cr-C` as it places more electron density between the metal and the binding carbon.