The asymmetric unit of the title salt contains two symmetrically independent [PW12O40]3– polyoxoanions, six L-isoleucinium (2S,3S) cations and nine solvent water molecules. The anion shows a classical a-Keggin structure, which consists of a central PO4 tetrahedron surrounded by four vertex-sharing W3O13 trimers. The cation is mono-protonated L-iso-leucine amino acid. Most of the N–H units and all O–H units of six cations are involved in hydrogen-bonding interactions as H-donors, and some of the terminal and bridged oxygen atoms of the anions and the carbonyl oxygen atoms of three cations act as H-acceptors. The water molecules are also involved in hydrogen-bonding interactions. The extensive intermolecular N–H…O and O–H…O hydrogen bonds make a three-dimensional hydrogen-bonded network in the crystal structure.

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