The complexes W(CO)3(PCy3)2(L) have been studied by solution calorimetry. The enthalpies of binding (kcal/mol) of ligands to W(PCy3)2(CO)3 in toluene solution are as follows: H2, -9.9, N2, -13.5; NCCH3, -15.1; pyridine, -18.9; P(OMe)3, -26.5; CO, -30.4. Similar values are obtained in tetrahydrofuran solution. These data imply bond strengths much lower than expected from gas-phase studies. The origin of this discrepancy is attributed to the presence of the W⋯H-C "agostic" interaction. This is estimated to be on the order of 10 ± 6 kcal/mol. In order to investigate the role of the "agostic" interaction in the energetics of this complex, the kinetics of reaction of W(CO)3[P(C6H11)3]2(py) and W(CO)3[P(C6D11)3]2(py) with P(OMe)3 in toluene were studied. A kinetic isotope effect, kH/kD = 1.20 ± 0.05, was observed, and verifies the importance of the "agostic" bond in ligand substitution for this complex. A mechanism is proposed that involves concerted replacement of coordinated pyridine by the three-center W⋯H-C bond.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry