Kinetics and Mechanism of Ligand-exchange Reactions in fac-(Piperidine )[ 1 ,2-bis( diphenylphospino )ethane ]tricarbony ltungsten(O)

Chemistry Department, College of Education, King Saud University
Manuscript received 27 July 1993, accepted 14 October 1993

The kinetics and mechanism of piperidine (=pip) displacement form fac-(pip)(diphos)W(C0)₃(=R) by a number of phosphine and phosphite ligands (=J) in chlorobenzene, toluene or acetone to afford fac-(L)(diphos)W(C0)₃ are reported. This thermal substitution process proceeds via a mechanism which involves reversible dissociation of piperidine form fac-(pip)(diphoso)W(CO)₃ (governed by the rate constants K₁ and K-₁) and followed by ligand attack at the resulting [(diphos)W(CO)₃] intermediate (governed by the rate constant K₂). This ligand-exchange reaction proceeds via a ligand -independent path and obeys a rate law, -d[R]/dt = K₁[R]. Rate constants and activation parameters in different solvents are reported.

Competitive reactions (pip vs L) for the [(diphos)W(CO)₃] intermediate are also reported. Although there is slight preference for ligands with small Tolman cone angle, the results indicate little discriminating ability for the [(diphos)W(CO)₃] intermediate upon combination with nucleophiles with different steric and electronic factors. The rate constant for W-pip dissociation K₁ exhibits a slight increase With increasing solvent polarity. However, the competition ratio, K₂IK-₁ seems to have minimal dependency on the polar character of the solvent.