Two nickel(II) complexes of formula (H(3)dien)(2)[Ni-2(ox)(5)]. 12H(2)O (1) and [Ni-2(dien)(2)(H2O)(2)(ox)]Cl-2 (2) (dien = diethylenetriamine and ox = oxalate dianion) have been synthesized and characterized by single-crystal X-ray diffraction. 1 crystallizes in the orthorhombic system, space group Abnn, with a 15.386(4) Angstrom, b = 15.710(4) Angstrom, c = 17.071(4) Angstrom, and Z = 4. 2 crystallizes in the monoclinic system, space group P2(1)/c, with a 10.579(1) Angstrom, b 7.258(1) Angstrom, c 13.326(1) Angstrom, beta = 93.52(3)degrees, and Z = 2. The structures of 1 and 2 consist of dinuclear oxalato-bridged nickel(II) units which contain bidentate oxalate (1) and tridentate dien in the fac-conformation (2) as terminal ligands. Both features, oxalato as a peripheral ligand and dien in the fac-conformation (instead of its usual mer-conformation), are unprecedented in the coordination chemistry of nickel(II). The nickel atom is six-coordinated in both compounds, the chromophores being NiO6 (1) and NiN3O3 (2) The Ni-O(ox) bond distances at the bridge (2.072(4) Angstrom in 1 and 2.11(1) and 2. 125(9) Angstrom in 2) are Somewhat longer than those concerning the terminal oxalate (2.037(5) and 2.035(3) Angstrom in 1). Magnetic susceptibility data of 1 and 2 in the temperature range 4.2-300 K show the occurrence of intramolecular antiferromagnetic coupling with J = -22.8 (1) and -28.8 (2) cm(-1) (J being the parameter of the exchange Hamiltonian H = -JS(A) . S-B). The observed value of -J in the investigated oxalato-bridged nickel(II) complexes, which can vary from 22 to 39 cm(-1), is strongly dependent on the nature of the donor atoms from the peripheral ligands. This influence has been analyzed and rationalized through extended Huckel calculations.