Complexes of the type [Ru(tpy)(L)(dmso)](n+) (where tpy = 2,2':6',2 ''-terpyridine; L = 2,2'-bipyridine (bpy), n = 2 N,N,N',N'-tetramethylethylene diamine (tmen), n = 2; acetylacetonate (acac), n = 1; oxalate (ox), n = 0; malonate (mal), n = 0) were investigated by density functional theory (DFT). The results do not support a promoting role for the d sigma* ligand field (LF) states during excited state S -> O isomerization. Instead, the calculations show that the formation of a Ru(III) center is important in the isomerization, along with the identity of the ancillary bidentate ligand. The present work shows that the orbital contributions from the bidentate ligand to the HOMO, which is typically centered on the ruthenium, plays an important role in the photochemical and oxidative reactivity of the complexes.