Access to unsaturated ruthenium complexes via phosphine complexation with triphenylborane: Synthesis and structure of a zwitterionic arene complex, (eta(6)-Ph-BPh2H)Ru(PMe3)(2)(SiMe3)

Triphenylborane, BPh3, serves as a phosphine "sponge", scavenging free PMe3 from alkane solutions of the 18e(-) complex (PMe3)(4)Ru(SiMe3)H to form sp aringly soluble Ph3B-PMe3 and the 16e(-) (PMe3)(3)Ru(SiMe3)H. Under nitroge n atmosphere the 16e(-) (PMe3)Ru-3(SiMe3)H forms a dimeric N-2 adduct, [(PM e3)(3)Ru(SiMe3)H](2)N-2. Both the 16e(-) complex and its N-2 adduct exist i n equilibrium with the 18e(-) silene complex, (PMe3)(3)Ru(CH2=SiMe2)H-2. Ho wever, long reaction times in the presence of excess borane leads to remova l of another phosphine ligand as Ph3B-PMe3 and formation of a new zwitterio nic complex, (eta(6)-PhBPh2H)Ru(PMe3)(2)(SiMe3) (5), in which a molecule of borane has abstracted a ruthenium hydride ligand and also coordinates as a n eta(6)-arene. The merits and Limitations of BPh3 as a phosphine removal a gent are discussed. Compound 5 has been characterized by single-crystal X-r ay analysis and exhibits unusually long Ru-C-arene bonds.