Nonlinear optical (NLO) properties of the clusters (Me.N)2[Cu4(SPh)6] (I) and (Me4N)2[Ag6(SPh)a] (II) were studied with a 7 ns pulsed liner at 532 nm. Both I and II exhibit NLO absorption and NLO refraction, which can be roughly described by third-order processes. A wide transparent window (λ=400-1000nm) was observed for both clusters, which leads to reintively large values of the figure of merit, W (=n2//(aoλ)|). The W value is 32 and 5 for I and If, respectively, at incident irredinnce of 0.3 GW cm-2. The nonlinear optical properties of the clusters were related to the extent of electron delocnlization of their HOMOs and LUMOs. Structurnl factors that may lead to optimization of linear and nonlinear optical of a material were discussed.