The preparation and properties are described of two related Ni-5(II) clusters. The reactions of [Ni(acac)(2)(H2O)(2)] (acacH = acetylacetone) with benzotriazole (btaH) and 5,6-dimethylbenzotriazole (5,6diMebtaH) in refluxing Me2CO in the presence of H2O leads to the isolation of [Ni-5(OH)(bta)(5)(acac)(4)(H2O)(4)] (1) and [Ni-5(OH)(5, 6diMebta)(5)(acac)(4)(H2O)(4)] (2) in 70-75 and 40-50% yields, respectively. Complex 1 . 4Me(2)CO . 0.5C(6)H(14) crystallizes in the triclinic space group P (1) over bar with (at 25 degrees C) a 13.885(1) Angstrom, b = 12.013(1) Angstrom, c = 25.611(2) A, alpha = 89.02(1)degrees, beta = 104.76(2)degrees gamma = 111.78(1)degrees, and Z = 2. Complex 2 . 4Me(2)CO crystallizes in the monoclinic space group C2/c with (at 25 degrees C) a = 19.085(3) Angstrom, b = 20.142(3) Angstrom, c = 22.574(4) Angstrom, beta = 103.30(1)degrees, and Z = 4. The Ni-II assemblies of 1 and 2 are composed of a tetrahedral arrangement of four six-coordinate metal ions centered on the fifth. Each of the five eta(1):eta(1):eta(1):mu(3) benzotriazolate ligands spans an edge of the Ni-4 tetrahedron. The OH- ion bridges three Ni-II ions and spans the sixth edge of the tetrahedron. A chelating acac(-) ion and a terminal H2O molecule complete the coordination sphere of each peripheral metal. Variable-temperature magnetic susceptibility data (5.0-295 K), obtained for 1 and 2, show antiferromagnetic interactions for both of them. The data are interpreted using a five-J model, which is based upon the hierarchy of algebras approach. Least-squares fitting of the data gives exchange parameters J for the interactions between the benzotriazolate-bridged peripheral Ni-II ions in the range from -3.9 to -10.7 cm(-1), and for those between the central ion and the peripheral ones in the range from -3.1 to -6.1 cm(-1); the J value for the interaction between the O-bridged peripheral nickels is -39.6 cm(-1) for 1 and -43.2 cm(-1) for 2. Magnetization data are in line with an intermediate spin ground state [S = 0, S = 1] for both clusters. An orbital interpretation of the coupling is proposed.