Bonding properties and electronic structures of mixed-ligand diethylenetriamine copper(II) complexes. Molecular structure of[Cu(biimidazole)(diethylenetriamine)](ClO4)(2)

Mixed-ligand diethylenetriamine copper(II) complexes of the type Cu(LL) (dien)(X), where dien is diethylenetriamine, LL is 2,2'-biimidazole (bimH(2)), 1,10-phenanthroline, 2,2'-bipyridine and ethylenediamine, and X is ClO4 and CF3SO3, have been synthesized and characterized by elemental analyses and electronic, vibrational and EPR spectroscopic measurements. The crystal and molecular structure of [Cu(bimH(2)) (dien)](ClO4)(2) has been determined using three-dimensional X-ray diffraction data. The copper(II) ion is coordinated by a diethylenetriamine and a biimidazole ligand, forming a distorted CuN5 square pyramid with one of the imidazole moieties bound at the apical position. The biimidazole is nearly perpendicular to the CuN4 basal plane with a bite angle of 78.6(5)degrees. Gaussian resolved d-d spectra for these square-pyramidal diethylenetriamine complexes yield a one-electron orbital sequence: d(x2-y2) >> d(z2) > d(xy) > d(xz) similar to d(yz). Bonding properties of these complexes are elucidated; biimidazole is a strong pi-acceptor, whilst 2,2'-bipyridine and 1,10-phenanthroline do not involve in pi-interactions.