A family of quaternary (or pseudoquaternary) antimonides Nb(4)Pd(0.5)ZSb(2) (Z = Cr, Fe, Co, Ni, Si) containing up to three transition metals in an ordered arrangement has been prepared by reactions of the elements. These antimonides are isostructural, crystallizing as substitutional variants of the W5Si3-type structure (tetragonal, space group D-4h(18)-I4/mcm, Z = 4) with unit cell parameters a = 10.4407(3) Angstrom and c = 5.0020(2) Angstrom for Nb4Pd0.5Cr0.28(3)-Si0.72Sb2, a = 10.4825(6) Angstrom and c = 4.9543(3) Angstrom for Nb4Pd0.5FeSb2, a = 10.4603(5) Angstrom and c = 4.9457(3) Angstrom for Nb4Pd0.5CoSb2, a = 10.4332(7) Angstrom and c = 4.9649(3) Angstrom for Nb4Pd0.5Ni0.78(1)Sb2, and a = 10.3895(10) Angstrom and c = 4.9634(4) Angstrom for Nb4Pd0.5SiSb2. They are distinguished by the filling of interstitial Z atoms into the centers of Nb-8 square antiprismatic clusters that are linked by PdSb4 tetrahedra. The Nb8 square antiprisms share opposite square faces to form one-dimensional chains along the c axis so that Z-Z bonding distances of similar to2.5 A result. Extended Huckel band structure calculations were carried out to interpret the homo-and heteroatomic metal-metal interactions in the structure. The resistivity of one member, Nb4Pd0.5SiSb2, was measured, indicating metallic behavior.