We have measured the third-order susceptibility, χ(3), versus concentration for a number of molybdenum-based metal-organic complexes in solution, using independent degenerate four-wave mixing and Z-scan techniques. Good agreement was obtained between the degenerate four-wave mixing and Z-scan measurements. The variation of χ(3) with concentration yielded the second-order hyperpolarizability γ. A close correlation was observed between the number of delocalized π electrons and the magnitude of γ. © 1994 Optical Society of America.