Keggin structure

It is the structural form of α-Keggin anions, which have a general formula of [XM12O40]n−, where X is the heteroatom (most commonly are pentavalent phosphorus PV, tetravalent silicon SiIV, or trivalent boron BIII), M is the addendum atom (most common are molybdenum Mo and tungsten W), and O represents oxygen.

Alfred Werner, using the coordination compounds ideas of Copaux, attempted to explain the structure of silicotungstic acid.

[citation needed] In 1928, Linus Pauling proposed a structure for α-Keggin anions consisting of a tetrahedral central ion, [XO4]n−8, caged by twelve WO6 octahedra.

[citation needed] James Fargher Keggin with the use of X-ray diffraction experimentally determined the structure of α-Keggin anions in 1934.

[2] = + The structure has full tetrahedral symmetry and is composed of one heteroatom surrounded by four oxygen atoms to form a tetrahedron.

The heteroatom is located centrally and caged by 12 octahedral MO6 units linked to one another by the neighboring oxygen atoms.

The metal centres in the 12 octahedra are arranged on a sphere almost equidistant from each other, in four M3O13 units, giving the complete structure an overall tetrahedral symmetry.

[5] Including the original Keggin structure there are 5 isomers, designated by the prefixes α-, β-, γ-, δ- and ε-.

Examples are the [XM11O39]n− and [XM9O34]n− formed by the removal from the Keggin structure of sufficient Mo and O atoms to eliminate 1 or 3 adjacent MO6 octahedra.

In 2015, the iron Keggin ion was isolated from water, but as a polyanion with a −17 charge; and protecting chemistry was required.

Depending on the solvent, acidity of the solution and the charge on the α-Keggin anion, it can be reversibly reduced in one- or multiple-electron steps.