The Distinctive Electronic Structures of Rhenium Tris(thiolate) Complexes, an Unexpected Contrast to the Valence Isoelectronic Ruthenium Tris(thiolate) Complexes

The noninnocent 2-diphenylphosphino-benzene-thiolate (DPPBT) ligand containing both phosphorus and sulfur donors delocalizes the electron density in a manner reminiscent of dithiolenes. The electronic structure of the [ReL3](n) (L = DPPBT, n = 0, 1+, 2+) complexes was probed with density-functional theory (DFT) and high-level ab initio methods including complete active space self-consistent field (CASSCF and CASPT2) and coupled cluster (CCSD and CCSD(T)). DFT predicts a slight preference for a dosed-shell (CS) singlet ground state for the neutral [ReL3] and stronger preferences for low-spin ground states for the oxidized [ReL3](+) and [ReL3](2+). High-level ab initio methods confirm a CS singlet with a Re(III) (d(4), S = 0) center as the ground state of [ReL3](0). Thus, this neutral Re species has considerably less thiyl radical character than the valence isoelectronic [RuL3](+), which is mainly a Ru(III) (d(5), S = 1/2) anti-ferromagnetically (AF) coupled to a thiyl radical (S = 1/2). However, the oxidized derivatives [ReL3](+) and [ReL3](+) show significant metal-stabilized thiyl radical character like [RuL3](+). Both [ReL3](2+) and [ReL3](+) have major contributions from Re(III) (d(4), S = 1) centers AF coupled to thiyl and dithiyl DPPBT ligands. These findings are consistent with the experimental chemistry as [RuL3](+), [ReL3](+), and [ReL3](2+) can add ethylene to form the new C-S bonds, but [ReL3](0) cannot. The thiyl radicals on the S2 position (the S trans to a P donor) serve as the intrinsic electron acceptors in the actual ethylene addition reactions with Ru and Re tris(thiolate) complexes.