An Unusual Asymmetric Polyoxomolybdate Containing Mixed-Valence Antimony and Its Derivatives: [Sb<sub>4</sub><sup>V</sup>Sb<sub>2</sub><sup>III</sup>Mo<sub>18</sub>O<sub>73</sub>(H<sub>2</sub>O)<sub>2</sub>]<sup>12−</sup> and {M(H<sub>2</sub>O)<sub>2</sub>[Sb<sub>4</sub><sup>V</sup>Sb<sub>2</sub><sup>III</sup>Mo<sub>18</sub>O<sub>73</sub>(H<sub>2</sub>O)<sub>2</sub>]<sub>2</sub>}<sup>22−</sup> (M = Mn<sup>II</sup>, Fe<sup>II</sup>, Cu<sup>II</sup> or Co<sup>II</sup>)

A new type of heteropolyanion containing mixed-valence antimony, [Sb<sub>4</sub><sup>V</sup>Sb<sub>2</sub><sup>III</sup>Mo<sub>18</sub>O<sub>73</sub>(H<sub>2</sub>O)<sub>2</sub>]<sup>12−</sup> (<b>1a</b>), and its four derivatives, {M(H<sub>2</sub>O)<sub>2</sub>[Sb<sub>4</sub><sup>V</sup>Sb<sub>2</sub><sup>III</sup>Mo<sub>18</sub>O<sub>73</sub>(H<sub>2</sub>O)<sub>2</sub>]<sub>2</sub>}<sup>22−</sup> (M = Mn<sup>II</sup>, Fe<sup>II</sup>, Cu<sup>II</sup>, or Co<sup>II</sup>) (<b>2a−5a</b>), have been isolated as ammonium salt, and their structures were determined by single-crystal X-ray diffraction. The framework of the polyanion <b>1a</b> displays a curious asymmetric structure, and there exist six types of Sb coordination environments and seven types of {MoO<sub>6</sub>} octahedra. The title compounds were also characterized by elemental analyses, IR, UV–vis, Raman spectra, and cyclic voltammogramms.