The preparation, single crystal X-ray crystallography, and magnetic properties are reported for four new clusters based on [M′^V(CN)₈]³⁻ octacyanometallates (M′ = Mo, W). Reactions of [M′^V(CN)₈]³⁻ with mononuclear Ni^II ions in the presence of the tmphen blocking ligand (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) in a 2 : 3 : 6 ratio, respectively, lead to the formation of the trigonal bipyramidal clusters [Ni^II(tmphen)₂]₃[M′^V(CN)₈]₂. Analogous reactions with the same starting materials performed in a 2 : 3 : 2 ratio, respectively, produce pentadecanuclear clusters of the type {Ni^II[Ni^II(tmphen)(MeOH)]₆[Ni(H₂O)₃]₂[μ-CN]₃₀[W^V(CN)₃]₆}. The W₂Ni₃ (1) and Mo₂Ni₃(2) pentanuclear clusters and the W₆Ni₉ (3) and Mo₆Ni₉ (4) pentadecanuclear molecules are isostructural to each other and crystallize in the space groupsP2₁/c and R respectively. Magnetic measurements indicate that the ground states for the trigonal bipyamidal clusters are S = 4 as a consequence of ferromagnetic coupling with J_W–Ni = 9.5 cm⁻¹, J_Mo–Ni = 10 cm⁻¹. The pentadecanuclear clusters exhibit ferromagnetic coupling as well, which leads to S = 12 ground states (J_W–Ni = 12 cm⁻¹, J_Mo–Ni = 12.2 cm⁻¹). Reduced magnetization studies on the W–Ni analogues support the conclusion that they exhibit a negative axial anisotropy term; the fits give D values of −0.24 cm⁻¹ for the W₂Ni₃cluster and D = −0.04 cm⁻¹for the W₆Ni₉cluster. AC susceptibility measurements indicate the beginning of an out-of-phase signal for the W₂Ni₃ and the W₆Ni₉ compounds, but detailed low temperature studies on small crystals by the microSQUID technique indicate that only the pentadecanuclear cluster exhibits hysteresis in accord with SMM behavior. Neither Mo cluster reveals any evidence for slow paramagnetic relaxation at low temperatures.