The static polarizabilities and the second-order hyperpolarizabilities of a series of tri-nuclear metal cluster models MS4(M(PPh3)2(M(PPh3) (M=Mo,W; M(=Cu, Ag, Au) have been calculated within the first-principle theoretical framework. The model clusters have two fragments of rhombic units and it is the charge transfer from one of these moieties to the other that is responsible for nonlinear optical property. This kind of electronic delocalization, differentiated from that of planar (-system, is very interesting and is worthy for further investigation.