How is backbonding treated in the theory of covalent bonding?

1 Answer
Aug 20, 2017

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


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.