Multicomponent Polyanions. 53. Structure of Tetrakis(trimethylammonium) Tetra-μ-oxo-bis(triaquahexadecaoxo(trioxophenylphosphato)hexamolybdate) Dihydrate, [NH(CH₃)₃]₄[{(C₆H₅P)Mo₆O₂₁(H₂O)₃}₂].2H₂O

The title compound crystallized in the monoclinic space group P2₁/n (No. 14) [a = 11.211 (5), b = 12.862 (3), c = 23.05 (1) Å, β = 94.37 (3)°, V = 3314 (2) ų, Z = 2]. The polyanion can be regarded as a dimer of a phenylphosphonatohexamolybdate, (C₆H₅P)Mo₆O₂₃(H₂O)₃, linked by four O atoms. In this monomeric unit the six molybdenum octahedra are grouped into two parts consisting of four and two edge-sharing octahedra, respectively. These two parts are connected by two corner-sharing O atoms to form a bent Mo₆ ring. The phenylphosphonate group coordinates to the Mo₆ ring from the narrow side as a tripodal ligand. The {(C₆H₅P)Mo₆O₂₁(H₂O)₃}₂ units form layers parallel to (001) and the structure is stabilized by hydrogen bonds between water and neighboring anions. The monomeric unit has been shown to be a key structure in the process of deducing the aqueous solution structures of the (C₆H₅P)Mo and (C₆H₅P)Mo species found in a previous equilibrium study of the H⁺–MoO–(C₆H₅P)O system.

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