An unexpected rhenium(IV)–rhenium(VII) salt: [Co(NH<sub>3</sub>)<sub>6</sub>]<sub>3</sub>[Re<sup>VII</sup>O<sub>4</sub>][Re<sup>IV</sup>F<sub>6</sub>]<sub>4</sub>·6H<sub>2</sub>O
Acta crystallographica. Section E, Crystallographic communications