The properties of isothiocyanato(3-thiapentane-1,5-dithiolato)oxorhenium(V) [ReO(SSS)NCS, (1a), (3+1) type], where isothiocyanato occupies the fifth position, have been studied. Two linkage isomers, i.e., ReO(SSS)NCS (1a) and ReO(SSS)SCN (1b), were found to be formed during syntheses. The sufficient quantities of la were isolated in the solid state, and characterized by X-ray crystallography and IR spectroscopy. From H-1 and N-15 NMR measurements, it was found that la is in equilibrium with 1b in liquid state. In the solvents with low dielectric constant (epsilon) such as CH2Cl2, only la isomer was detected, while in the solvents with high 9 such as CH3CN, both la and 1b isomers were observed. We have obtained the equilibrium constant (K-iso) for the linkage isomerization reaction in CD3CN by measuring N-15 NMR spectra at various temperatures. The values of K-iso at 25 degrees C the standard enthalpy (Delta H degrees), and the standard entropy (Delta S degrees) for the isomerization equilibrium were evaluated as 0.409, 14.4 kJ mol(-1), and 40.9 J K-1 mol(-1), respectively. (c) 2006 Elsevier B.V. All rights reserved.