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

Abstract

The noninnocent 2-diphenylphosphino-benzenethiolate (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 closed-shell (CS) singlet ground state for the neutral [ReL₃]⁰ 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) (d⁴, 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) (d⁵, S = 1/2) anti-ferromagnetically (AF) coupled to a thiyl radical (S = 1/2). However, the oxidized derivatives [ReL₃]⁺ and [ReL₃]²⁺ show significant metal-stabilized thiyl radical character like [RuL₃]⁺. Both [ReL₃]⁺ and [ReL₃]²⁺ have major contributions from Re(III) (d⁴, 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.