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A Keggin unit (also known as a Keggin structure) is the naturally occurring form of various oxo-metallic acids.  The acids in question are not simple acids like HCl or HBr, but complex chemical acids that contain a transition metal like molybdinum or vanadium, oxygens, acidic hydrogens, and a heteroatom like P or Si.  These types of acids will self assemble in aqueous solution to form Keggin structures.  The Keggin structure consists of a central heteroatom bonded to four oxygen atoms.  This central chemical species is surrounded by a cage-like structure consisting of twelve repeating units of the transition metal bonded to six separate oxygen atoms.  This complex structure is drawn in the link below.  Chemist J.F. Keggin first elucidated this structure in 1934 using x-ray diffraction analysis.

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