Roles of a tetrahydroborate ligand in a facile route to ruthenium(II) ethyl hydride complexes, and a kinetic study of ethane reductive elimination

The tetrahydroborate ligand in Ru(eta(2)-BH(4))(CO)H(PMe(2)Ph)(2)], 1, allows conversion under very mild conditions to Ru(CO)(Et)H(PMe(2)Ph)(3)], 7, by way of Ru(eta(2)-BH(4))(CO)Et(PMe(2)Ph)(2)], 4. Deprotection of the hydride ligand in 7(by BH(3) abstraction) occurs only in the final step, thus preventing premature ethane elimination. A deviation from the route from 4 to 7 yields Ru(eta(2)-BH(4))(COEt)(PMe(2)Ph)(3)], 6, but does not prevent ultimate conversion to 7. Modification of the treatment of 4 yields an isomer of 7, 10. Both isomers eliminate ethane at temperatures above 250 K: the immediate product of elimination, thought to be Ru(CO)(PMe(2)Ph)(3)], 11, can be trapped as Ru(CO)(PMe(2)Ph)(4)], 12, Ru(CO)H(2)(PMe(2)Ph)(3)], 3a, or Ru(CO)(Ctriple bond]CCMe(3))H(PMe(2)Ph)(3)], 13. The elimination is a simple first-order process with negative DeltaS(++) and (for 7) a normal kinetic isotope effect (k(H)/k(D)= 2.5 at 287.9 K). These results, coupled with labelling studies, rule out a rapid equilibrium with a sigma]-ethane intermediate prior to ethane loss.