Molybdenum(VI) complexation by amino acids Part II. Species in aqueous solutions of ( R)-cysteine and sodium molybdate or tungstate

Abstract

Complex formation between Mo( VI) and ( R)- cysteine, abbreviated H 2cys, was studied in aqueous solutions ( C Mo⩽0.1 mol dm −3), using circular dichroism (CD) spectrophotometry and NMR spectrometry. Only one Mo(VI) complex is present in neutral or basic solutions, [MoO 3(cys)] 2− ( I), in which cysteine is bonded to the metal as a terdentate ligand. Its formation constant was found to be log K=18.8±0.2, at T=18±1 °C and I=1 mol dm −3. In solutions with pH values between 4.5 and 6.5–7.0 at least one other complex species II is formed, in addition to complex I. Complex II is believed to have the same stoichiometric ratio L/M = 1 and to be formed by reaction of complex I with acid. All solutions were unstable, the time necessary for disappearance of the Mo(VI) complexes varying with concentration, C L/ C Mo and pH. A single complex [WO 3(cys)] 2− is formed in aqueous solutions of sodium tungstate and ( R)- cysteine at pH > 6.7, and its formation constant was determined by CD spectrophotometry. The influence of the method used in the preparation of the solutions on the reactions of W(VI)with ( R)-cysteine is reported. Circular dichroism, 1H and 13C NMR spectra of the Mo(V) complex [Mo 2O 4(cys) 2] 2− and its aquation products are also presented and discussed.