By using eta(3)-(SCH2CH2SCH2CH2S) (SSS), 2-thiocytosine (HL1) and 5-amino-1,3,4-thiadiazole-2-thiol (HL2) as S-donor ligands two '3 + 1' mixed ligand red oxorhenium(V) complexes of the type [ReO(SSS)(HL1)] Cl (1Cl) and [ReO(SSS)(L-2)] (2) have been synthesized. Both 1Cl and 2 have been characterized by C, H and N microanalyses, H-1 NMR, FT-IR, UV-Vis spectra and conductivity measurements. The X-ray crystal structures of both 1Cl and 2 have been determined. Electrochemical studies of 1Cl in acetonitrile show four successive one-electron redox couples; while 2 in DMSO displays only two one-electron redox steps. A computational and conceptual Density Functional Theory (DFT) studies have been performed on various (3 + 1) complexes of Re(V). The fully-optimized complexes, 1Cl and 2, adopt geometries that satisfactorily corroborate the X-ray structures. Subsequently these optimized structures were employed to investigate systematically the relative stabilities of various Re(V) complexes with varying ligands having N, O and S donor atoms. The electronic properties of 1Cl and 2 were delved into employing the hybrid STMP TD-DFT method and the results were found to be in good harmony with the experimental observations.