A series of organic-inorganic hybrid materials of the copper(II)-molybdophosphonate family have been prepared using conventional hydrothermal conditions. The reactions of MoO3, copper(II) acetate, bipyrimidine (bpyr), a phosphonic acid, and water at temperatures below 160 degrees C and in the presence of a mineralizer such as acetic acid or HF provided crystalline samples of materials of the general class {Cu-2(bpyr)}(4+)/MoxOy-phosphonate. The recurrent themes of the structures are the presence of the binuclear {Cu-2(bpyr)}(4+) and pentanuclear {Mo5O15(O3PR)(2)}(4-) building blocks. For the alkylphosphonate-containing materials, [{Cu-2(bpyr)(2)}Mo5O15(O3PCH3)(2)]center dot 2.5H(2)O (1 center dot 2.5H(2)O) is two-dimensional and exhibits {Cu(bpyr)}(n)(2n+) chains, while [{Cu-2(bpyr)(H2O)}Mo5O15(O3PCH2CH3)(2)] (2) is three-dimensional. The diphosphonate series of materials {{Cu-2(bpyr)}(4+)[Mo5O15{O3P(CH2)(n)PO3}](4-) with n = 2-6 (4, 5, 7-9) in all cases contain the characteristic [Mo5O15{O-3-P(CH2)(n)PO3}](n)(2n+) chains, linked through {Cu-2(bpyr)}(4+) rods into three-dimensional frameworks. When n = 1, the three-dimensional phase [{Cu-2(bpyr)}MoO2(HO3PCH2PO3)(2)]center dot 2H(2)O (3 center dot 2H(2)O) is obtained, the exclusive example of a structure constructed from isolated {MoO6} polyhedra rather than pentamolybdate clusters. The Ni(II)-containing phase [{Ni-2(bpyr)-(H2O)(4)}Mo5O15{O3P(CH2)(3)PO3}]center dot 9H(2)O (6 center dot 9H(2)O) was also prepared and compared to the structure of the Cu(II) analogue, [{Cu-2(bpyr)(H2O)(4)}Mo5O15{O3P(CH2)(3)PO3}]center dot 3H(2)O (5 center dot H2O)- Magnetic susceptibility studies of the compounds revealed that the magnetic behavior was consistent in all cases with antiferromagnetically coupled dimers. However, the magnitude of the exchange coupling was clearly dependent on the orientation of the M(II) mean equatorial or basal planes relative to the bipyrimidine plane. Thus, when the metal and bipyrimidine planes are nearly coplanar, the J values are in the -77 to -87 cm(-1) range, while J values of -2 to -5 cm(-1) are observed for the compounds with out-of-plane orientations.