Two mononuclear complexes of formula (Et3BzN)2[ReCl6] (1) and (Et3BzN)2[PtCl6] (2) (Et3BzN+ = triethylbenzylammonium cation) have been synthesized in good yield (70 and 75 %, respectively) and structurally characterized. From a structural point of view, they both crystallized in the monoclinic centrosymmetric P21/n space group and the two molecular structures are almost perfectly superimposable, with only minor differences in the M–Cl bond lengths. Solid‐state direct‐current (dc) magnetic susceptibility analysis in the temperature range 1.9–300 K for 1 reveal the occurrence of magnetically isolated rhenium(IV) ions with a significant zero‐field splitting (zfs), D = +6.7 cm–1 (2D being the energy gap between the MS = ±3/2 and MS = ±1/2 Kramers doublets). DFT, CASSCF and CASSCF/NEVPT2 theoretical calculations were carried out on the experimental geometry of the complex [ReCl6]2– in 1 to substantiate the parameters defining the zfs tensor. Alternating current (ac) magnetic susceptibility measurements of 1 in the temperature 2.0–10 K show incipient frequency–dependence of the out‐of‐phase signals below 3.0 K under non‐zero applied dc fields supporting a field–dependent Single Ion Single Molecule Magnet (SISMM) behaviour of this rhenium(IV) derivative.