Heterolytic Activation of Dihydrogen at Transition‐Metal Sulfur Sites in Coordinatively Unsaturated [rh(L)(“ bu S 4 ”)]BF 4 Complexes, Involving Neutral Hydrides, Thiol Hydrides, and Thiol–Hydride Proton Scrambling (L = CO, PCy 3 ; “ bu S 4 ” 2‐ = 1,2‐Bis[(2‐mercapto‐3,5‐di‐ tert ‐butylphenylthio]ethane 2‐ )

[Rh(H)(L)(“ bu S 4 ”)] complexes (L = CO ( 1 ), PCy 3 ( 2 ); “ bu S 4 ” 2‐ = 1,2‐bis[(2‐mercapto‐3,5‐di‐ tert ‐butylphenylthio)ethane 2‐ ]) catalyze the D 2 /H + exchange between D 2 and EtOH protons in the presence of catalytic amounts of Brønsted acids. A mechanism and complete cycle for the heterolytic D 2 cleavage are proposed that are based on characterization of key intermediates and monitoring of key reactions. The key intermediates are the thiol hydride complexes [Rh(H)(L)(“ bu S 4 ”‐H)]BF 4 , L = CO ( 3 ), PCy 3 ( 4 ), the coordinatively unsaturated complexes [Rh(L)(“ bu S 4 ”)]BF 4 , L = CO ( 5 ), PCy 3 ( 6 ), which are the actual catalysts, and the deuterium‐labeled derivatives of 1–4 . Complexes 3 and 4 form from 1 and 2 by protonation with HBF 4 , and they release H 2 to give 5 and 6 . Complex 5 dimerizes in the solid state and was characterized by X‐ray structure determination of 5 ·8CH 2 Cl 2 (triclinic space group P \documentclass{article}\pagestyle{empty}\begin{document}$ \bar 1 $\end{document} , a = 1048.2(4) pm, b = 1430.0(5) pm, c = 1785.7(7) pm, α = 100.49(3)°, β = 102.92(3)°; γ = 103.68(3)°, Z = 1). Complex 6 is mononuclear and adds H 2 O or THF reversibly to give the highly labile [Rh(L)(PCy 3 )(“ bu S 4 ”)]BF 4 , L = H 2 O ( 7 ), THF ( 8 ). CO is irreversibly added to give the stable [Rh(CO)‐(PCy 3 )(“ bu S 4 ”)]BF 4 ( 9 ), whose high‐frequency ν(CO) (2081 cm ‐1 ) indicates a relatively low electron density at the Rh center. Complex 6 also adds to H 2 to give 4 , which can be deprotonated by solid Na 2 CO 3 or H 2 O to yield neutral 2 . 1 H NMR and 2 H NMR spectroscopy revealed the scrambling of thiol protons and hydride ligands in 3 and 4 and its deuterium‐labeled derivatives. This exchange of thiol protons for hydride ligands is explained by a transient [Rh(η 2 ‐H 2 )] species. Low‐temperature 1 H/ 2 H NMR spectroscopy showed that protonation of 2 yields four diastereomers of 4 resulting from protonation of the four stereochemically nonequivalent lone pairs at the thiolate donors of 2 . The relevance of these findings to H 2 activation at transitionmetal sulfur sites in hydrogenases or hydrotreatment catalysts, and differences from the H 2 cleavage achieved with other complexes not containing “built‐in” Brønsted‐basic centers, are discussed.