Syntheses and Reactivity of Cationic Borane−Ruthenium Complexes [(η⁵-C₅R₅)Ru(PMe₃)₂(η¹-BH₃·EMe₃)][BAr<i>^f</i>₄] (R = H, Me; E = N, P; BAr<i>^f</i><sub>4</sub

Chloride displacement from [(η5-C5R5)Ru(PMe3)2Cl] by Na[BArf4] in the presence of BH3·EMe3 afforded cationic borane σ complexes containing an unsupported B−H−Ru bridge, [(η5-C5R5)Ru(PMe3)2(η1-BH3·EMe3)][BArf4] (4a: R = H, EMe3 = PMe3; 4b: R = Me, EMe3 = PMe3; 4c: R = H, EMe3 = NMe3; [BArf4] = [B{3,5-C6H3(CF3)2}4]). Compounds 4a−c were fully characterized, and their structures were determined by X-ray crystallographic analysis. In these complexes, the borane ligand is coordinated to the ruthenium atom through a B−H−Ru three-center two-electron bond. They exhibit fluxional behavior due to site exchange between the BH hydrogen atoms. Complexes 4a and 4c are remarkably stable. However, they are hydrolyzed by a trace amount of water to produce a cationic dihydride, trans-[CpRuH2(PMe3)2]+ (6, Cp = η5-C5H5). The reactivity of 4a toward various substrates was investigated.