Three isostructural compounds of the formula {[M(bipy)(H2O)(H2P2O7)](2)center dot 2H(2)O} [bipy = 2,2'-bipyridine; M = Ni (1), Co (2), Mn (3)] have been isolated from aqueous solutions containing the corresponding metal(II) chloride hydrate with a bipy and sodium pyrophosphate solution in a 1:1:2 molar ratio, and their structures were determined by single-crystal X-ray diffraction. The structures of 1-3 consist of neutral aqua(2,2'-bipyridine)metal(II) dinuclear units bridged by two dihydrogenpyrophosphate groups adopting a bidentate/monodentate mode. Each metal ion in 1-3 is six-coordinate in a distorted octahedral geometry, with the reduced value of the angle subtended by the chelating bipy at the metal ion [79.6(1)degrees (1), 77.32(7)degrees (2), and 72.9(1)degrees (3)] being the main source of this distortion. The values of the intramolecular metal-metal separation are 5.271(1) angstrom (1), 5.3065(8) angstrom (2), and 5.371(1) angstrom (3). Magnetic susceptibility measurements on polycrystalline samples of 1-3 in the temperature range 1.9-300 K shows weak intramolecular ferromagnetic [J = +1.86(2) cm(-1) (1) and +0.25(1) cm(-1) (2)] and antiferromagnetic [j = -0.48(1) cm-1 (3)] coupling, with the spin Hamiltonian being defined as H = -JS(M1)center dot S-M1a. This rarely observed coordination mode for dihydrogenpyrophosphate leads to ferromagnetic coupling in complexes of nickel(II) or cobalt(II).