[Mo154O462H14(H2O)70]14−. The Mo154-type cluster was then shown to be the basic structural type of molybdenum blue compounds obtained under slightly different conditions. The structure of the big wheel is constructed from units containing 11 Mo atoms ({Mo11}-type units), 14 of which are linked together to form the {Mo154}-type cluster that has an external diameter of 3.4 nm. (12 {Mo11}-type units are also involved in the construction of higher symmetrical spherical systems called Keplerates) These units consist of a central MoO7 bipyramid sharing edges with 5 MoO6 octahedra (an illustration of this is on page 155 of the review ). With 5 more linking MoO6 octahedra the repeating {Mo11}-type unit is built up. The spherical vesicle Along with other aggregates, a hollow, spherical structure self-assembles from approximately 1,165 Mo154 wheels. This was termed a vesicle by analogy with lipid vesicles. Unlike lipid vesicles that are stabilised by hydrophobic interactions it is believed that the vesicle is stabilised by an interplay of van der Waals attraction, long-range electrostatic repulsion with further stabilization arising from hydrogen bonding involving water molecules encapsulated between the wheel-shaped clusters and in the vesicles' interior. The radius of the vesicle is 45 nm. Molybdenum blue pigment A pigment termed molybdenum blue is recorded in 1844 as a mixture of molybdenum with "oxyde of tin or phosphate of lime". An alternative formulation involves "digesting" molybdenum sulfide with nitric acid to form molybdic acid, which is then mixed with tin filings and a little muriatic acid (HCl). This is evaporated and heated with alumina. A 1955 paper states that molybdenum blue is unstable and is not used commercially as a pigment. The chemistry of these pigments has not been investigated. References Mixed valence compounds Molybdenum compounds Cluster chemistry Heteropoly acids Chemical tests