Compounds of W(VI) with l(−)malic acid: a polarimetric and 1H nuclear magnetic resonance study of the formation and interconversion equilibria in excess of W(VI)

Abstract

Complex formation occurring between l(−)malic acid and W(VI) has been investigated by polarimetric and cryoscopic measurements, and also by proton nmr spectroscopy. The complexing ability of malic acid depends on the metal–ligand ratio. With excess W(VI), four different complexes are formed. The pH of the medium is the principal variable controlling complex formation and the various interconversion equilibria. At high pH ( > 7.0), the stable complex is a monomer, while at lower pH values dimers predominate. Conditional dissociation constants of some species have been evaluated and they show that the complexes formed with excess W(VI) have a higher stability than those found when malic acid is in excess. Comparison of the proton coupling constants shows that the conformation of malic acid in its metal complexes depends on which reagent is in excess, and leads to a number of conclusions about the structure of the dimeric species.