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 Mo^VI in 2–6 mol dm^–3 nitric or perchloric acid solutions. This species has characteristic vibrational frequencies ν_sym(Mo–O) 952, ν_asym(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 Mo^VI 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.