Molybdenum(V) Solutions in Hydrobromic Acid

Interest in the coordination chemistry of molybdenum has increased greatly in recent years because of the discovery that molybdenum plays an active role in several redox enzymes. Model systems have been devised to gain insight into the function of molybdenum in these enzymes. Most model systems are believed to contain an oxomolybdate(V) dimer in equilibrium with a redox active monomer or dimer. A relatively simple system that shows important similarities to model systems is a molybdenum(V) solution in hydrobromic or hydrochloric acid. As in the model systems, these solutions are believed to contain monomers and dimers in equilibrium. The equilibria are highly dependent on acid concentration, and to a lesser extent, on molybdenum(V) concentration. Identification of the molybdenum(V) species in these solutions would be helpful in determining the structures of species in model systems. But there is considerable disagreement concerning the molybdenum(V) species in these solutions. Different methods of attack, though apparently valid, have led to different conclusions. A study of oxomolybdate solutions in hydrobromic acid was undertaken to resolve some of the major disagreements about the identities of the molybdenum(V) species. Hydrobromic acid solutions were studied, because less work has been done with them than with the hydrochloric acid solutions. Another reason for using hydrobromic acid solutions was the greater sensitivity of the electronic absorption spectrum and electron spin resonance spectrum to changes in structure in concentrated acid. Also of interest were the reported tendencies of some of the molybdenum(V) species in hydrobromic acid to polymerize. A more comprehensive spectrophotometric study was necessary to observe this phenomenon. Electronic absorption and electron spin resonance spectra of oxobromomolybdate(V) compounds were used for comparison with hydrobromic acid solution spectra. Conclusions concerning structures of molybdenum(V) species in hydrobromic acid were based largely on such comparisons