Spectroscopic and electrochemical studies of chromium(III) complexes with some naturally occurring ligands containing sulphur

Several Cr(III) complexes of L(-)cystine, L(-)cysteine, dibenzyldisulphide, alpha-mercaptosuccinic acid and 5,5'-dithiobisnitrobenzoic acid were prepared and characterized by their vibrational, electronic and circular dichroism measurements. The coordination of sulphur-containing amino acids involved amino and carboxylate groups. In the visible spectra, two peaks were observed at 15 900-18 700 and 23 000-25 300 cm(-1) and were assigned to (4)A(2g)-->T-4(2g) and T-4(2g)-->T-4(1g) (f) d-d transitions, respectively in octahedral (O-h) or pseudo-octahedral (D-3) symmetry. Three sharp Cotton effects were observed in the circular dichroism spectra of cysteine and cystine chelates and were assigned to the (2)E(2E(g)), (2)A(2)(T-2(1g)), and (2)E(T-2(1g)) d-d transitions. The observed Cotton effects in the allowed band arise from the splitting of (4)A(2g)-->T-4(2g) to (4)A(1)(T-4(2g)) and (4)E(T-4(2g)) transition. The parameters D-q, B, beta(35) and the variation of interelectronic repulsion value with ionic charge, Z* for the complexes were calculated. The magnetic moments (3.6-3.9 B.M) are close to the spin-only value for a d(3) Cr(III) ion in octahedral symmetry. Cyclic voltammetry experiments on some of the complexes revealed a reversible Cr(III)/Cr(II) redox couple and pronounced solvent and scan rate effects on the peak potentials and peak currents.