Nonlinear optical properties of transition metal-phosphine complexes h
ave been measured at 532 nm by degenerate four-wave mixing. Large nonr
esonant second-order molecular hyperpolarizabilities, gamma, have been
found for complexes containing two phosphine ligands. The measured ga
mma values are closely related to the type and coordination geometry o
f the phosphine ligands. A numerical fitting of gamma values versus th
e number of substituents with pi-electrons on the ligand gives a resul
t similar to what is observed for linear conjugated oligomers.