Syntheses and reactivity of cationic borane-ruthenium complexes [(η5-C5R5)Ru(PMe3)2(η1-BH3•EMe3)][BArf4] (R = H, Me; E = N, P; BArf4=[B{3,5-C6H3(CF3)2}4])

Chloride displacement from [(eta(5)-C5R5)Ru(PMe3)(2)Cl] by Na[BAr4f] in the presence of (BH3EMe3)-E-. afforded cationic borane sigma complexes containing an unsupported B-H-Ru bridge, [(eta(5)-C5R5)Ru(PMe3)(2)(eta(1)-(BH3EMe3)-E-.)][BAr4f] (4a: R = H, EMe3 = PMe3; 4b: R = Me, EMe3 = PMe3; 4c: R = H, EMe3 = NMe3; [BAr4f] = [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 = eta(5)-C5H5). The reactivity of 4a toward various substrates was investigated.