Electronic and bonding properties of mixed-ligand pyridine-2,6-dicarboxylate copper(II) complexes: Molecular structures of [Cu(4-dimethylaminopyridine)(pyridine-2,6-dicarboxylato)] and [Cu(N-methylimidazole)(pyridine-2,6-dicarboxylato)(0.5H(2)O)]

Mixed-ligand pyridine-2,6-dicarboxylatocopper(II) complexes. [Cu(dipic)(L)] [dipic = pyridine-2,6-dicarboxylate, L = imidazole, N-methylimidazole (nmim), 4-methylimidazole, pyridine, 4-methylpyridine, 4-aminopyridine, 4-dimethylamino-pyridine(4dmapy) and NH3 . H2O], have been synthesized and characterized by elemental analyses, and UV-vis, IR and EPR spectroscopic measurements. Molecular structures of [Cu(dipic)(4dmapy)] (1) and [Cu(dipic)(nmim)(H2O)(0.5)] (2) have been determined by X-ray diffraction methods. Complex 1 is square planar and 2 comprises both square pyramidal [Cu(dipic)(nmim)(H2O)] and square planar [Cu(dipic)(nmim)] units. The equatorial heterocyclic unidentate ligands are nearly co-planar with the CuN2O2 coordination plane : 1.3 degrees for 1, 6.8 degrees and 13.9 degrees for the square plane and square pyramid, respectively, of 2. The structures of other heterocyclic complexes are suggested to be square planar based on their spectroscopic data. The sequence of d orbitals was assigned as d(x)2(-y)2 >> d(xz) > d(xy) > d(yz) > d(z)2 for the square planar complexes and d(x)2(-y)2 >> d(z)2 > d(xz) > d(yz) for the square pyramidal NH3 complex. The significant rise in energy of the d(xz) orbitals suggests that the central pyridine nucleus of the dipicolinate ligand and the heterocyclic unidentate ligands are pi-donors. This is attributable to ionic bonding effects. Copyright (C) 1996 Elsevier Science Ltd.