| Summary: | Two mononuclear Re<sup>IV</sup> complexes of general formula (PPh<sub>4</sub>)<sub>2</sub>[ReX<sub>6</sub>] [PPh<sub>4</sub><sup>+</sup> = tetraphenylphosphonium cation, X = Br (<b>1</b>) and I (<b>2</b>)] have been prepared and structurally and magnetically characterised. Both compounds crystallise in the triclinic system with space group <i>P</i>ī. Their structures are made up of hexahalorhenate(IV), [ReX<sub>6</sub>]<sup>2−</sup>, anions, and bulky PPh<sub>4</sub><sup>+</sup> cations. Each Re<sup>IV</sup> ion in <b>1</b> and <b>2</b> is six-coordinate and bonded to six halide ions in a quasi regular octahedral geometry. In their crystal packing, the [ReX<sub>6</sub>]<sup>2−</sup> anions are well separated from each other through the organic cations, generating alternated anionic and cationic layers, and no intermolecular Re−X···X−Re interactions are present. Variable-temperature dc magnetic susceptibility measurements performed on microcrystalline samples of <b>1</b> and <b>2</b> show a very similar magnetic behaviour, which is typical of noninteracting mononuclear Re<sup>IV</sup> complexes with <i>S</i> = 3/2. Ac magnetic susceptibility measurements reveal the slow relaxation of the magnetisation in the presence of external dc fields for <b>1</b> and <b>2</b>, hence indicating the occurrence of the field-induced single-ion magnet (SIM) phenomenon in these hexabromo- and hexaiodorhenate(IV) complexes.
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