The synthesis, structure and catalytic properties of the [Mo <sub>7</sub> O <sub>24</sub> (μ-Mo <sub>8</sub> O <sub>26</sub> )Mo <sub>7</sub> O <sub>24</sub> ] <sup>16−</sup> anion formed <i>via</i> two intermediate heptamolybdates [Co(en) <sub>3</sub> ] <sub>2</sub> [NaMo <sub>7</sub> O <sub>24</sub> ]Cl· <i>n</i> H <sub>2</sub> O and (H <sub>3</sub> O)[Co(en) <sub>3</sub> ] <sub>2</sub> [Mo <sub>7</sub> O <sub>24</sub> ]Cl·9H <sub>2</sub> O