The real and imaginary parts of third order nonlinear optical susceptibilities (chi (3)) of 10–4M solutions of [{Fe2(CO)6}(mgr 3-Y3P){CpCr(CO)2}] (Y=S (1) or Se (2)) and the well known precursor clusters [Fe3(CO)9(mgr 3-Y)2] (Y=S (3) or Se (4)) in toluene were measured using Z-scan and ARINS techniques respectively. Compounds 1 and 2 possess nearly three times the gamma R values of their corresponding precursor compounds, 3 and 4. The results suggest a rich potential of mixed-metal, mixed-nonmetal class of clusters as materials exhibiting large nonlinearity.