Monosubstituted Borane Ruthenium Complexes RuH₂(η²:η²‑H₂BR)(PR′₃)₂: A General Approach to the Geminal Bis(σ-B–H) Coordination Mode

A series of borane bis­(σ-B–H) ruthenium complexes RuH₂(η²:η²-H₂BR)­(PR′₃)₂ (R = alkyl, aryl; R′ = Cy, Cyp, ^<i>i</i>Pr) has been prepared by using two synthetic strategies. The first one is based on a simple substitution reaction by adding the corresponding monosubstituted H₂BR borane to the bis­(dihydrogen) ruthenium complex RuH₂(η²-H₂)₂(PCy₃)₂. The second one, more general, results from the reaction of the chloro complex RuHCl­(H₂)­(PR′₃)₂ (R′ = Cy, Cyp, ^<i>i</i>Pr) with the corresponding lithium monosubstituted borohydrides RBH₃Li (R = Mes, ^<i>t</i>Bu, Me, C₄H₃S, Ph). All the complexes have been characterized by multinuclear NMR, IR, and X-ray diffraction studies. DFT calculations have been used to better define the bonding mode of the borane ligand to the metal center as well as to establish the thermodynamic cycle that delineates the coordination process. The ^<i>t</i>Bu species displays a dynamic behavior evidencing an equilibrium between a borohydride and a σ-borane formulation. The thienyl case illustrates the competition between sulfur coordination and a bis­(σ-B–H) coordination mode