Reaction of (NH4)(2)WO2S2, CuBr, and Et(4)NBr in solid state produced (NEt(4))(3)[WOS3(CuBr)]. 2H(2)O. Single-crystal X-ray diffraction data reveal that the anionic cluster [WOS3(CuBr)(3)(mu-Br)](3-) has a half-open cage structure, an intermediate structure between cubic-cage and nest structures. Nonlinear optical (NLO) properties of the cluster were studied with a 7-ns pulsed laser at 532 nm. The cluster exhibits both optical self-focusing and optical nonlinear absorption stemed from a fluence dependent process (n(2)(eff) = 1.1 x 10(-16) m(2) W-1, alpha(2)(eff) = 6.0 x 10(-10) m W-1, when measured with a 9.1 x 10(-4) mol dm(-3) acetonitrile solution). The corresponding effective NLO susceptibilities (chi(eff)((3))) of the cluster is 2.6 x 10(-10) esu. These nonlinear optical properties of the cluster were compared with those of cubic-cage-shaped and nest-shaped clusters and those of other known NLO materials.