Ruthenium(II) tris(pyrazolyl)borate complexes. Reversible vinylidene complex formation

A convenient high-yield route to [Ru{HB(pz)3}Cl(PPh3)(=C-CHR)] (pt = pyrazolyl; R = Ph,SiMe3, Bu-u, Bu-t, CO2Et or C6H9) has been found through the intermediary [Ru{HB(pz)(3)}(PPh3)(dmf)] (dmf = dimethyl- formamide) which has been crystallographically characterized. This complex is readily obtained on treatment of [Ru{HB(pz)(3)}Cl(cod)] (cod = cycloocta-1,5-diene) with 1 equivalent of PPh3 in boiling dmf. The vinylidene moiety in complexes of the type [Ru{HB(pz)(3)}Cl(PPh3)(=C-CHR)] is remarkably labile being easily replaced by nucleophiles L = PMe3, PPh3, MeCN, pyridine or CO to give [Ru{HB(pz)(3)}Cl(PPh3)L]. With the exceptions of L = PMe3 or CO, these reactions are reversible. The complex [Ru{HB(pz)(3)}Cl(PPh3)(CO)] has been characterized by X-ray crystallography. On treatment of [Ru{HB(pz)(3)}Cl(PPh3)(=C-CHPh)] with an excess of HC=CR' (R' = SiMe3, Bu-n, Bu-t, CO2Et or C6H9) the neutral vinylidene complex [Ru{HB(pz)(3)}Cl(PPh3)(=C=CHR')] was reversibly formed. Comparative studies and extended-Huckel molecular orbital calculations have been performed to elucidate the nature of the bonding in the various vinylidene complexes. The bonding between the metal center and the vinylidene ligand is dominated by the d(yz) (metal)-p (vinylidene) interaction If this contribution is small, reconversion of the vinylidene complex into an eta(2)-alkyne complex can be accomplished.