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A new tungstoboric species, [BW 13 O 46 H 3 ] 8-, which was identified in solution, is an intermediate between [BW 11 O 39 H] 8and the so-called hexagonal acid. This heteropolyacid was obtained in high yield by a new synthetic route. A single-crystal X-ray diffraction study showed that its formula is H 21 [B 3 W 39 O 132 ]‚69H 2 O. The heteropolyanion is formed by three BW 13 O 46 subunits linked together by sharing vertices leading to C 3h symmetry. Each subunit is constituted by a undecatungstic anion derived from the Keggin structure, capped by a µ-oxoditungstic group, W 2 O 11 . The BO 4 tetrahedron is significantly distorted, one of the B-O bonds being longer (1.58 Å) than the three others (1.46 Å). The conversions between the three species [BW 11 O 39 H] 8-, [BW 13 O 46 H 3 ] 8and [B 3 W 39 O 132 ] 21are rapid and reversible. On the contrary, the irreversible formation of the Keggin heteropolyanion, R-[BW 12 O 40 ] 5-, is observed by heating of the solution of [BW 13 O 46 H 3 ] 8at pH 1-5.

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