In an attempt to crystallize a new diphosphonate complex of polyoxomol
ybdate at pH 7, colorless crystals of the caesium salt of heptamolybda
te, Cs-6[Mo7O24].7H(2)O, were obtained. The structure of the heptamoly
bdate anion was first reported by Lindqvist [Arkiv Kemi (1950), 2, 325
-341] and subsequently confirmed by other groups. The structure report
ed here is the first of a caesium salt of heptamolybdate; the coordina
tion of the caesium counterions is of interest.