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 (Ea = 16.4 ± 0.6 kcal mol-1, ?H† = 16.0 ± 0.6 kcal mol-1, and ?S† = 12.7 ± 2.5 eu), H (Ea = 10.7 ± 0.2 kcal mol-1, ?H † = 10.3 ± 0.2 kcal mol-1, and ?S † = -7.2 ± 0.8 eu), and Cl (Ea = 16.3 ± 0.2 kcal mol-1, ?H† = 15.7 ± 0.2 kcal mol-1, and ?S† = -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, ?H † = 18.5 ± 0.3 kcal mol-1, and ?S † = 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)]. © 2010 American Chemical Society.