The polymolybdates and polytungstates are derived, formally at least, from the dianionic [MO] precursors. The most common units for polymolybdates and polyoxotungstates are the octahedral {MO} centers, sometimes slightly distorted. Some polymolybdates contain pentagonal bipyramidal units. These building blocks are found in the molybdenum blues, which are mixed valence compounds.