The spectroscopically observable tris(thiolate) complex [Ru(dppbt)(3)](+)(1(+)) (dppbt = diphenyl-phosphinobenzenethiolate) is reported to have chemistry based on thiyl-radical character. High-level ab initio methods predict the ground-state electronic structure of 1(+) to be an open-shell diradical singlet state with antiferromagnetic coupling between (S = 1/2) Ru(III) and (S = 1/2) S p(z), rather the previous description based on a diradical state involving two S p orbitals. These new results provide an improved understanding of the experimental chemistry of 1(+) and related species.