COMPLEX-FORMATION BETWEEN CROCONATE (C5O5(2-)) AND CUIIL [L = 2,2'-BIPYRIDINE(BIPY), 2,2'/6',2'-TERPYRIDINE OR BIS(2-PYRIDYLCARBONYL)AMIDE ANION] IN DIMETHYL-SULFOXIDE SOLUTION - CRYSTAL-STRUCTURE OF [CU(BIPY) (C5O5) (H2O)]

A new mononuclear complex of formula [Cu(bipy)(C5O5)(H2O)] [bipy = 2,2'-bipyridine, C5O52- = dianion of croconic acid (4,5-dihydroxycyclopent-4-ene-1,2,3-trione)] has been synthesised and characterized by spectroscopic and X-ray diffraction methods. It crystallizes in the triclinic space group P1BAR, with a = 7.1016(9), b = 9.3392(9), c = 11.8911(7) angstrom, alpha = 68.939(6), beta = 84.665(7), gamma = 68.974(9)-degrees and Z = 2. The structure consists of neutral monomeric [Cu(bipy)(C5O5)(H2O)] entities. The co-ordination geometry around each copper(II) ion is distorted square pyramidal with the two nitrogen atoms of bipy and two croconate oxygen atoms in the basal plane and a water oxygen atom at the apex. The protonation of croconate and its complex formation with Cu(II)L [L = bipy, 2,2':6'2"-terpyridine or bis(2-pyridylcarbonyl)amide anion] have been investigated in dimethyl sulfoxide solution at 25-degrees-C and with 0.1 mol dm-3 tetrabutylammonium perchlorate as background electrolyte. The co-ordination modes of croconate for this system are discussed in the light of both thermodynamic and structural parameters and compared to those of the related oxalato and squarato (3,4-dihydroxycyclobut-3-ene-1,2-dionato) complexes.