Syntheses, crystal structures and magnetic properties of [Ni-2(C2O4)-(tren)(2)][ClO4](2) and [Ni-2(C4O4)(tren)(2)(H2O)(2)][ClO4](2)[tren equals tris(2-amino-ethyl)amine]

Two dinuclear nickel(II) complexes [Ni-2(C2O4)(tren)(2)][ClO4](2) 1 and [Ni-2(C2O4)(tren)(2)(H2O)(2)][ClO4](2) 2 [tren = tris-(2-aminoethyl)amine, C2O42- = oxalate dianion and C4O42- = dianion of 3,4-dihydroxycyclobut-3-en; 1,2-dione (squaric acid)] have been synthesized and characterized by single-crystal X-ray diffraction. Their structures consist of dinuclear nickel(II) cations and unco-ordinated perchlorate anions. The nickel environment is distorted octahedral NiN4O2 in both complexes: four nitrogen atoms of the tren group acting as a tetradentate ligand and two oxygen atoms of oxalate (1) or. water molecule and a squarate oxygen (2) comprise the co-ordination. The oxalate acts as a bis(chelating) ligand whereas the squarate adopts a mu-1,2-bis(monodentate) co-ordination mode. The intradimer metal-metal separation is 5.413(1) and 6.224(1) Angstrom in 1 and 2, respectively. The co-ordinated water molecule in 2 forms an intramolecular hydrogen bond to an unco-ordinated squarate-oxygen atom. Variable-temperature susceptibility measurements revealed the occurrence of relatively strong(1) and weak (2) intramolecular antiferromagnetic coupling, the relevant parameters being J = -28.8 cm(-1) and g = 2.16 for 1 and J = -0.4 cm(-1) and g = 2.15 for 2 (J being the exchange parameter in the isotropic spin Hamiltonian H = -J<(S)over cap A>.<(S)over cap B>). A comparative study of the ability of oxalate and squarate groups to mediate electronic interactions in the structurally characterized oxalato- and squarato-bridged nickel(II) complexes was carried out.