Access to Unsaturated Ruthenium Complexes via Phosphine Complexation with Triphenylborane:  Synthesis and Structure of a Zwitterionic Arene Complex, (η⁶-Ph-BPh₂H)Ru(PMe₃)₂(SiMe₃)

Triphenylborane, BPh 3, serves as a phosphine “sponge”, scavenging free PMe 3 from alkane solutions of the 18e - complex (PMe 3 ) 4 Ru(SiMe 3 )H to form sparingly soluble Ph 3 B−PMe 3 and the 16e - (PMe 3 ) 3 Ru(SiMe 3 )H. Under nitrogen atmosphere the 16e - (PMe 3 ) 3 Ru(SiMe 3 )H forms a dimeric N 2 adduct, [(PMe 3 ) 3 Ru(SiMe 3 )H] 2 N 2 . Both the 16e - complex and its N 2 adduct exist in equilibrium with the 18e - silene complex, (PMe 3 ) 3 Ru(CH 2 SiMe 2 )H 2 . However, long reaction times in the presence of excess borane leads to removal of another phosphine ligand as Ph 3 B−PMe 3 and formation of a new zwitterionic complex, (η 6 -PhBPh 2 H)Ru(PMe 3 ) 2 (SiMe 3 ) ( 5 ), in which a molecule of borane has abstracted a ruthenium hydride ligand and also coordinates as an η 6 -arene. The merits and limitations of BPh 3 as a phosphine removal agent are discussed. Compound 5 has been characterized by single-crystal X-ray analysis and exhibits unusually long Ru−C arene bonds.