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 t2g orbital and a ligand π* orbital. 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 ligand is thus acting as a σ-donor and a π-acceptor. In π-backbonding, the metal donates π electrons to the ligand π* orbital, adding electron density to an antibonding molecular orbital. 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.