Molybdenum chemistry in molten LiCl-KCl eutectic: an electrochemical and absorption spectroscopy study of the concentration dependent stability of solutions of K3MoCl6

The chemical behaviour of LiCl-KCl-K3MoCl6 solutions between 400 and 500 de grees C was investigated. They underwent slow decomposition, according to a concentration dependent disproportionation reaction yielding metallic moly bdenum and a volatile black higher valency molybdenum compound, which corre sponded with MoCl5, Chronopotentiometric determinations showed a single 3-e lectron reduction, yielding Mo. The process was slow with diffusion-control led mass transfer. The concentration of the Mo(III) species, presumed in th e form of MoC63- ions, was determined using linear sweep voltametry and ope n circuit voltage measurements and the overall Mo(III) species concentratio n by chemical analysis. The results showed MoC63- to be the predominant spe cies in the melt and the presence of a previously proposed dinuclear specie s, Mo2Cl93-, could not be confirmed electrochemically. This arose to quanti fication errors in the determination of the electroactive species MoCl63- d ue to inherent problems in estimating the active surface area of the electr ode. The electronic absorption spectrum of MoCl63- in LiCl-KCl eutectic at 400 degrees C is reported for the first time. Measurements were made above and below 0.045 mol dm(-3) K3MoCl6, the concentration above which the dispr oportionation reaction was relatively slow. The results are discussed in de tail and the spectroscopic parameters of the octahedral MoC63- complex were calculated as: Dq = 1775 cm(-1), B = 511 cm(-1) and C = 1913 cm(-1). Altho ugh an additional band, at 340 nm, appeared during disproportionation it co uld not be identified with any currently known polynuclear molybdenum chlor o complex or oxychloro complex. It is probably associated with an intermedi ate in the disproportionation process. No spectral evidence was thus obtain ed in support of the formation of dinuclear molybdenum species. (C) 1999 El sevier Science Ltd. All rights reserved.