Redox-stimulated intramolecular isomerization of the DMSO ligand in [Os(bpy)(2)(DMSO)(2)](2+) (bpy = 2,2'-bipyridine; DMSO = dimethyl sulfoxide) was explored theoretically for better understanding the electrochromic properties of osmium sulfoxide complexes. It is found that the HOMOLUMO gap is decreased because the electron transfer amount from DMSO1 ligand to Os center using Os-S-1 linkage is larger than that using Os-O-1 linkage, which makes the absorption of such electrochromic Os(II) sulfoxide complexes red-shifted. Moreover, it is observed that Os-O linkage is preferred by the hard Os(III) metal and the soft Os(I) metal prefers Os-S linkage, compared with Os(II). Intrinsic reaction pathway calculation results demonstrate that Os-S1 -> O-s-O-1 isomerization is favored by Os(II) oxidation, while Os-O-1 -> O-s-S-1 isomerization is much easier to be triggered by reduction of Os(III) or Os(II). In addition, DMSO2 linkage isomerization becomes much harder to proceed attributed to the increased bond-strength between DMSO2 and Os center upon Os(II)-O-1 -> Os(II)-S-1 rearrangement, which makes only one DMSO ligand isomerized observed experimentally.