Molybdenum-95 nuclear magnetic resonance and vibrational spectroscopic studies of molybdenum(VI) species in aqueous solutions and solvent extracts from hydrochloric and hydrobromic acid: evidence for the complexes [Mo2O5(H2O)6]2+, [MoO2X2(H2O)2](X = Cl or Br), and [MoO2Cl4]2?

Abstract

Aqueous molybdate solutions which contain a series of polymeric oxomolybdenum(VI) complex ions have been investigated conjointly by 95Mo and 17O n.m.r. and Raman spectroscopy. The spectra provide evidence of the species [Mo7O24]6–, [Mo8O26]4–, [Mo36O112(H2O)16]8– believed to be formed successively prior to the point at which hydrous MoO3(‘molybdic acid’) precipitates from the acidified solution. This solid will redissolve and a further complex, probably [Mo2O5(H2O)6]2+, is the predominant form of MoVI in 2–6 mol dm–3 nitric or perchloric acid solutions. This species has characteristic vibrational frequencies νsym(Mo–O) 952, νasym(Mo–O) 925, and ν(Mo–O–Mo) 830 cm–1, and gives a 95Mo n.m.r. signal at δ–63 p.p.m. In halogen acid solutions additional complexes exist and spectroscopic evidence shows these to be polymeric oxohalogenomolybdenum(VI) species and the discrete complexes [MoO2X2(H2O)2](X = Cl or Br). Oxygenated organic solvents efficiently extract MoVI from halogen acid solutions in this form and the vibrational spectra of ether or ketone phases show the [MoO2X2(H2O)2] complexes to have cis-trans-cis disposition of ligands. Characteristic 95Mo chemical shifts of the halogeno-complexes [MoO2X2(H2O)2] occur at δ 157 (X = Cl), 217 (X = Br), and 187 p.p.m. for the mixed halide [MoO2ClBr(H2O)2]. I.r. and Raman spectra of the cis-[MoO2Cl4]2– anion have been observed and assigned by studying the crystalline Cs+ salt, but this complex is not present in significant amounts in molybdenum(VI) solutions in concentrated hydrochloric acid.