Here the phosphine ligand acts as a σ-donor and a π-acceptor, forming a dπ-dπ bond. The backbonding in this case is analogous to the CO example, except that the acceptor orbital is a phosphorus 3d orbital rather than a ligand π* orbital. This is a common situation for phosphine complexes (e.g., triphenylphosphine) bound to low-valent, late transition metals. This results in weakening of the C-O bond, which is experimentally observed as lengthening of the bond (relative to free CO in the gas phase) and lowering of the C-O infrared stretching frequency.ĭ-d π bonding occurs when an element such phosphorus, which has a σ-symmetry lone pair and an empty metal 3d orbital, binds to a metal that has electrons in a t 2g orbital. In π-backbonding, the metal donates π electrons to the ligand π* orbital, adding electron density to an antibonding molecular orbital. The ligand is thus acting as a σ-donor and a π-acceptor. This situation is called " back-bonding" because the ligand donates σ-electron density to the metal and the metal donates π-electron density to the ligand. The most common situation is when a ligand such as carbon monoxide or cyanide donates its sigma (nonbonding) electrons to the metal, while accepting electron density from the metal through overlap of a metal t 2g orbital and a ligand π* orbital. There are three types of pi-bonding in metal complexes: \)Īn important factor that contributes to the high ligand field strength of ligands such as CO, CN -, and phosphines is π-bonding between the metal and the ligand.
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