The crystal of (NH4)4S2Mo5O21·3H2O contains a discrete polyanion, S2Mo5O214−, formed by reactions in an aqueous solution: 5Mo7O246−+14SO2+2H+7S2Mo5O214−+H2O or 5MoO42−+2SO2+6H+2Mo5O214−+3H2O It is orthorhombic, with the space group Pbcn and with a=15.096(9), b=13.375(8), c=23.734(7) Å, Dm=2.79, and Dx=2.79g cm−3 for Z=8. The crystal consists of S2Mo5O214− anions connected with each other by hydrogen bonds, with intervening water molecules and ammonium cations. In the polyanion, five MoO6 octahedra are connected by edge sharing, and the two octahedra at the two ends are connected by corner sharing to complete the Mo5O21 ring; the two SO3 trigonal pyramids are attached to the ring from above and below the ring.
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Matsumoto et al. (1976) studied this question.