Keggin structure
Keggin structure is the best known structural form for heteropoly acids. 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 P5+, Si4+, or B3+), M is the addenda atom (most common are molybdenum and tungsten), and O represents oxygen. The structure self-assembles in acidic aqueous solution and is the most stable structure of polyoxometalate catalysts.
The first α-Keggin anion, ammonium phosphomolybdate ((NH4)3[PMo12O40]), was first reported by Berzelius in 1826. In 1892, Blomstrand proposed the structure of phosphomolybdic acid and other poly-acids as a chain or ring configuration. Alfred Werner, using the coordination compounds ideas of Copaux, attempted to explain the structure of silicotungstic acid. He assumed a central group, [SiO4]4− ion, enclosed by four [RW2O6]+, where R is a unipositive ion. The [RW2O6]+ are linked to the central group by primary valences. Two more R2W2O7 groups were linked to the central group by secondary valences. This proposal accounted for the characteristics of most poly-acids, but not all.
In 1928, Linus Pauling proposed a structure for α-Keggin anions consisting of a tetrahedral central ion, [XO4]n−8, caged by twelve WO6 octahedral. In this proposed structure, three of the oxygen on each of the octahedral shared electrons with three neighboring octahedral. As a result, 18 oxygen atoms were used as bridging atoms between the metal atoms. The remaining oxygen atoms bonded to a proton. This structure explained many characteristics that were observed such as basicities of alkali metal salts and the hydrated of some of the salts. However the structure could not explain the structure of dehydrated acids.
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