Nonlinear optical properties of transition metal-phosphine complexes have been measured at 532 nm by degenerate four-wave mixing. Large nonresonant second-order molecular hyperpolarizabilities, γ, have been found for complexes containing two phosphine ligands. The measured γ values are closely related to the type and coordination geometry of the phosphine ligands. A numerical fitting of γ values versus the number of substituents with π-electrons on the ligand gives a result similar to what is observed for linear conjugated oligomers. © 1995.