Molybdenum(VI) complexes with malic acid: their inter-relationships, and the crystal structure of dicaesium bis[(S)-malato(2–)]-cis-dioxomolybdate(VI)–water (1/1)

Abstract

The complex anions obtainable in salts crystallising from aqueous solutions of molybdate and malic acid (H₃mal) are of the types (i)[MoO₂(Hmal)₂]^2–, (ia)[MoO₂(mal)₂]^4–, and (ii)[Mo₄O₁₁(mal)₂]^4– The formation of each type is dependent primarily upon the reactant ratio, M⁺ : Mo : H₃mal. With the structure of the type (ii) anion already known, the structure of the (simpler) anion (i) was determined by X-ray analysis of its salt, Cs₂[MoO₂(Hmal)₂]·H₂O. The complex is mononuclear, with each malato-ligand co-ordinated through the deprotonated hydroxy group and the vicinal carboxy group to form a five-membered chelate ring. The second carboxylate group is not co-ordinated, but there is inter-anion H-bonding between the two carboxy functions. The structural features of class (i) and (ii) anions show the importance of ligand characteristics in determining the type of molybdenum-oxygen core which forms. Compositional and i.r. evidence relating to class (ia) indicates it to be derived from (i) through proton replacement.