SUBSTITUTION OF A NITRILE BY P-DONOR LIGANDS IN [RE3(MU-H)3(CO)11(NCME)] AND AXIAL-EQUATORIAL ISOMERIZATION OF THE ENTERING LIGANDS - A KINETIC-STUDY

The substitution of the axially coordinated nitrile by P-donor ligands [PPh3, PPh2Me, PPhMe2, PMe3, P(n-Bu)3, P(OMe)3] in the triangular cluster complex [Re3(mu-H)3(CO)11(NCMe)] (1) gives axial derivatives 2, which then transform reversibly into the equatorial isomers 3. The kinetics of both substitution and isomerization reactions have been studied, by using the hydridic region of H-1 NMR spectra to evaluate the concentrations of the three complexes. The data for the substitution have been accounted for by a reversible dissociative mechanism, with rate r = (k1k2[1][PR3])/(k-1[MeCN] + k2[PR3]). The constant k1, which is independent of the nature of the entering ligand, was evaluated under pseudo-first-order conditions, at five temperatures, allowing the estimation of the activation parameters (E(a) = 118.3 +/- 1.8 kJ mol-1, DELTA-H double-ended daggar = 115.9 +/- 1.7 kJ mol-1, DELTA-S double-ended daggar = 70.3 +/- 5.6 J K-1 mol-1). The values of the competition ratios k2/k-1 for the six ligands have been determined at 300 K (range 0.12-0.91). For the three smaller ligands, ln k2 increases linearly with the sigma-donicity (measured by the pK(a) of HPR3+) and the steric profile of the reaction shows that k2 decreases for ligands with cone angles higher than about 125-130-degrees (steric threshold) The kinetic constants k3 for the axial-equatorial isomerization (obtained by computer optimization) have values between 8.3 x 10(-6) and 4.7 x 10(-5) s-1 (300 K) and depend on both steric and electronic parameters, increasing with the sigma-basicity and the steric hindrance of the ligands. The activation parameters have been estimated for PPh2Me and for PPh3. The equilibrium constants K(e) = k3/k-3 (evaluated from the equilibrium ratios [3]/[2], range 1.9-8.7) do not vary significantly with the temperature (292-314 K) and are affected mainly by electronic factors, increasing with the sigma-basicity of the ligands.