Polythiophenes exhibit interesting third-order nonlinear optical properties but poor solubilities. Incorporating polythiophenes into transition metal complexes should yield readily soluble materials, and the transition metal substituent may alter third-order nonlinear optical properties of the polythiophene. In this paper, we report the synthesis and characterization of 2-diphenylphosphinothianaphthene, A, and its trans-PdCl 2L 2, B, and cis-PtCl 2L 2, C, analogues. Additionally 2-diphenylphoshino-5:2′,5′:2″-terthiophene, D, its trans-PdCl 2L 2 complex, E, both first prepared by Wolf, 25 the previously unprepared cis-PtCl 2L 2 analogue, F, and the oxide of A, G, were prepared and investigated for nonlinear optical (NLO) behavior. No appreciable third-order NLO behavior was observed for A, D, or G. However both E and F exhibited nonlinear refraction, as shown by Z-scan measurements, and nonlinear absorption. The nonlinear absorption was significantly stronger for E, indicating that the metal center has a significant affect on nonlinear absorption. This is the first reported observation of this type of nonlinear behavior in metal-phosphinothiophene systems. © 2006 American Chemical Society.