Ferromagnetic Coupling in “Double-Bridged” Dihydrogenpyrophosphate Complexes of Cobalt(II) and Nickel(II)

Three isostructural compounds of the formula {[M(bipy)(H2O)(H2P2O7)]2·2H2O} [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)° (1), 77.32(7)° (2), and 72.9(1)° (3)] being the main source of this distortion. The values of the intramolecular metal–metal separation are 5.271(1) A (1), 5.3065(8) A (2), and 5.371(1) A (3). Magnetic susceptibility measurements on polycrystalline samples of 1–3 in the temperature range 1.9–300 K shows ...