A mixed ligand oxido–rhenium(V) complex, [ReOS3(HL)]Cl.H2O (1pCl.H2O), with 3-thiopentane-1,5-dithiolato (S3) as a tridentate ligand and imidazolidinethione (HL) as an ancillary monodentate sulfur donor co-ligand, has been synthesized. 1pCl.H2O has been characterized by spectral analyses. The X-ray crystal structure of 1pCl.H2O shows that the complex contains a distorted square-pyramidal “ReOS4” core. The structural parameters agree with our optimized structure of 1p+. Subsequently, the optimized structure was used to calculate systematically the relative stabilities of a sequence of oxido–Re(V) and the analogous oxido–Tc(V) complexes just by varying the donor sites (N, S, and O) on the tridentate ligand moiety in 1p+. Electrochemical studies on 1pCl.H2O show an oxidative rhenium(VI)/ rhenium(V) couple at 1.561 V versus Ag/AgCl under controlled linear diffusion situation. Vibrational frequencies, electronic structures, and redox potential of 1p+ have been calculated theoretically employing density functional theory (DFT) or time-dependent-DFT methods. The experimental findings are in excellent agreement with the computed results. The calculated redox potentials of the investigated oxido–Re(V) complexes and their oxido–Tc(V) counterparts are shown to correlate linearly with their respective chemical potential values. © 2018 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim