A series of Ni(II) carboxylate complexes, supported by a chelate ligand having either secondary hydrophobic phenyl groups (6-Ph(2)TPA, N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine) or hydrogen bond donors (bnpapa, N,N-bis((6-neopentylamino-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine), have been prepared and characterized. X-ray crystallographic studies of [(6-Ph(2)TPA)Ni(O2C(CH2)(2)SCH3)]ClO4 center dot CH2Cl2 (4 center dot CH2Cl2) and [(6-Ph(2)TPA)Ni(O2CCH2SCH3)]ClO4 center dot 1.5CH(2)Cl(2) (5 center dot 1.5CH(2)Cl(2)) revealed that each complex contains a distorted octahedral Ni(II) center and a bidentate carboxylate ligand. A previously described benzoate complex ([(6-Ph(2)TPA)Ni(O2CPh)]ClO4 (3)) has similar structural characteristics. Recrystallization of dry powdered samples of 3, 4 center dot 0.5CH(2)Cl(2), and 5 from wet organic solvents yielded a second series of crystalline Ni(II) carboxylate complexes having a coordinated monodentate carboxylate ligand ([(6-Ph(2)TPA)Ni(H2O)(O2CPh)]ClO4 (6), [(6-Ph(2)TPA)Ni(H2O)(O2C(CH2)(2)SCH3)]ClO4 center dot 0.2CH(2)Cl(2) (7 center dot 0.2CH(2)Cl(2)), [(6-Ph(2)TPA)Ni(H2O)(O2CCH2SCH3)]ClO4 (8)) which is stabilized by a hydrogen-bonding interaction with a Ni(II)-bound water molecule. In the cationic portions of 7 center dot 0.2CH(2)Cl(2) and 8, weak CH/pi interactions are also present between the methylene units of the carboxylate ligands and the phenyl appendages of the 6-Ph(2)TPA ligands. A formate complex of the formulation [(6-Ph(2)TPA)Ni(H2O)(O2CH)]ClO4 (9) was isolated and characterized. The mononuclear Ni(II) carboxylate complexes [(bnpapa)Ni(O2CPh)]ClO4 (10), [(bnpapa)Ni(O2C(CH2)(2)SCH3)]ClO4 (11), [(bnpapa)Ni(O2CCH2SCH3)]ClO4 (12), and [(bnpapa)Ni(O2CH)]ClO4 (13) were isolated and characterized. Two crystalline solvate forms of 10(10 center dot CH3CN and 10 center dot CH2Cl2) were examined by X-ray crystallography. In both, the distorted octahedral Ni(II) center is ligated by a bidentate benzoate ligand, one Ni(II)-bound oxygen atom of which accepts two hydrogen bonds from the supporting bnpapa chelate ligand. Spectroscopic studies of 10-13 suggest that all contain a bidentate carboxylate ligand, even after exposure to water. The combined results of this work enable the formulation of a proposed pathway for carboxylate product release from the active site Ni(II) center in acireductone dioxygenase.