Molybdenum Blue - Isopoly Molybdenum Blues

Isopoly Molybdenum Blues

The isopoly-molybdenum blues have been known for many years. They are the cause of the "blue waters" found near Idaho Springs, known to Native Americans. They were first documented by Scheele and Berzelius. The compounds responsible for the blue colour were not known until 1995. Before then it was well known that there were polymolybdates of Mo(VI). Molybdenum(VI)oxide, MoO₃, when dissolved in aqueous alkali forms the tetrahedral molybdate anion, MoO₄2−. Dissolving molybdate salts in strong acid produces "molybdic acid", MoO₃.2H₂O. In between these extremes of pH polymeric ions are produced which are mostly built from MoO₆ octahedral units sharing corners and edges. Examples include Mo₇O₂₄6−, Mo₈O₂₆4− and Mo₃₆O₁₁₂(H₂O)₁₆8− which contain the {(Mo)Mo₅}-type unit comprising a central MoO₇ pentagonal bipyramid sharing edges with 5 MoO₆ octahedra. The later unit occurs also in the giant mixed-valence molybdenum blue species 48− (x≈16) as well as in the cluster described in the next section. The molybdenum blue species are obtained by reduction of acidifed molybdate(VI) solutions.