The reaction in water of M(II) [M = Ni or Mn] with 1,10-phenanthroline (phen) and sodium pyrophosphate (Na4P2O7) in a 2:4:1 stoichiometry resulted in the crystallization of dinuclear complexes featuring the heretofore rare bridging pyrophosphate. Single-crystal X-ray diffraction studies revealed the complexes to be {[(phen)(2)Ni](2)(mu-P2O7)}center dot 27H(2)O (1) and {[(phen)(2)Mn](2)(mu-P2O7)}center dot 13H(2)O (2) where the asymmetric M(phen)(2) units are bridged by bis-bidentate pyrophosphate, each metal ion exhibiting a distorted octahedral geometry. The bridging pyrophosphate places adjacent metal centers at 5.031 angstrom in 1 and 4.700 angstrom in 2, and its conformation also gives rise to an intramolecular pi-pi interaction between two adjacent phen ligands. Intermolecular pi-pi interactions between phen ligands from adjacent dinuclear complexes create an ornate 3D network in 1, whereas a 2D sheet results in 2. The hydrophilic nature of the pyrophosphate ligand leads to heavy hydration with the potential solvent-accessible area for 1 and 2 accounting for 45.7% and 26.4% of their unit cell volumes, respectively. Variable-temperature magnetic susceptibility measurements on polycrystalline samples of 1 and 2 revealed net weak intramolecular antiferromagnetic coupling between metal centers in both compounds with J = -3.77 cm(-1) in 1 and J = -0.88 cm(-1) in 2, the Hamiltonian being defined as H = -JS(A)center dot S-B. The ability of the bis-bidentate pyrophosphate to mediate magnetic interactions between divalent first row transition metal ions is discussed bearing in mind the number and nature of the interacting magnetic orbitals.