Fluxionality of [(Ph₃P)₃M(X)] (M = Rh, Ir). the red and orange forms of [(Ph₃P)₃Ir(Cl)]. Which phosphine dissociates faster from wilkinson's catalyst?

NMR studies of intramolecular exchange in [(Ph₃P) ₃Rh(X)] (X = CF₃, CH₃, H, Ph, Cl) have produced full sets of activation parameters for X = CH₃ (E_a = 16.4 ± 0.6 kcal mol^-1, ΔH^† = 16.0 ± 0.6 kcal mol^-1, and ΔS^† = 12.7 ± 2.5 eu), H (E_a = 10.7 ± 0.2 kcal mol^-1, ΔH ^† = 10.3 ± 0.2 kcal mol^-1, and ΔS ^† = -7.2 ± 0.8 eu), and Cl (E_a = 16.3 ± 0.2 kcal mol^-1, ΔH^† = 15.7 ± 0.2 kcal mol^-1, and ΔS^† = -0.8 ± 0.8 eu). Computational studies have shown that for strong trans influence ligands (X = H, Me, Ph, CF₃), the rearrangement occurs via a near-trigonal transition state that is made more accessible by bulkier ligands and strongly donating X. The exceedingly fast exchange in novel [(Ph₃P) ₃Rh(CF₃)] (12.1 s^-1 at -100 °C) is due to strong electron donation from the CF₃ ligand to Rh, as demonstrated by computed charge distributions. For weaker donors X, this transition state is insufficiently stabilized, and hence intramolecular exchange in [(Ph ₃P)₃Rh(Cl)] proceeds via a different, spin-crossover mechanism involving triplet, distorted-tetrahedral [(Ph₃P) ₃Rh(Cl)] as an intermediate. Simultaneous intermolecular exchange of [(Ph₃P)₃Rh(Cl)] with free PPh₃ (THF) via a dissociative mechanism occurs exclusively from the sites cis to Cl (E _a = 19.0 ± 0.3 kcal mol^-1, ΔH ^† = 18.5 ± 0.3 kcal mol^-1, and ΔS ^† = 4.4 ± 0.9 eu). Similar exchange processes are much slower for [(Ph₃P)₃Ir(Cl)] which has been found to exist in orange and red crystallographic forms isostructural with those of [(Ph ₃P)₃Rh(Cl)].