The Mn(II) (1) and Ni(II) (2) members of the family [M4(H2O)2(P2W15O56)2]16- (M = Co(II), Cu(II), Zn(II)) have been synthesized and characterized by X-ray single-crystal analysis (for 1) and magnetic measurements. These compounds are isostructural to the Cu(II) and Zn(II) derivatives (Na16[Mn4(H2O)2(P2W15O56)2].53H2O (1), triclinic, P1BAR, a = 14.31(3) angstrom, b = 14.675(4) angstrom, c = 20.363(9) angstrom, alpha = 83.44(3)-degrees, beta = 80.61(7)-degrees, gamma = 73.85(6)-degrees, Z = 1; Na16[Ni4-(H2O)2(P2W15O56)2].52H2O (2), triclinic, P1BAR, a = 13.73(2) angstrom, b = 13.79(1) angstrom, c, = 21.52(7) angstrom, alpha = 90.7(2)-degrees, beta = 96.5(2)-degrees, gamma = 119.1(1)-degrees, Z = 1) and comprise a rhomblike M4O16 group (M = Mn(II), Ni(II)) encapsulated between two fragments of the trivacant Dawson-Wells polyanion [P2W15O56]12-. The crystal structure of the Mn(II) derivative has been solved and compared with that of the Cu(II) and Zn(II) derivatives. The Mn(II) derivative exhibits antiferromagnetic Mn-Mn exchange interactions (J = -0.7 cm-, J’ = -0.2 cm-1) and an S = 0 ground state. In turn, the Ni(II) derivative shows ferromagnetic exchange interactions within the Ni4O16 entity (J = 8.3 cm-1, J’ = 3.5 cm-1) and an S = 4 ground state, the highest spin state reported in a heteropoly complex. These magnetic properties are discussed in connection with the structure and compared with those of tetranuclear clusters of manganese and nickel, as well as with the copper derivative of the same series.