Fluxionality of [(Ph ₃P) ₃Rh(X)]: The extreme case of X = CF ₃

[(Ph ₃P) ₃Rh(F)] reacts with CF ₃SiMe ₃ to produce trans-[(Ph ₃P) ₂Rh(CF ₂)(F)] (1; X-ray), which is equilibrated with a number of species in solution. Addition of excess Ph ₃P shifts all of the equilibria to [(Ph ₃P) ₃Rh(CF ₃)] (2; X-ray) as the only NMR-observable and isolable (84%) species. Complex 2 is uniquely highly fluxional in solution, maintaining ligand exchange even at -100°C (12.1 s ^-1). Activation parameters have been determined (variable- temperature ³¹P NMR) for the similar but slower exchange in the Me analogue of 2, [(Ph ₃P) ₃Rh(CH ₃)]: E _a = 16.5 ± 0.6 kcal mol ^-1, ΔG _† = 12.9 kcal mol ^-1 (calculated at -30°C), ΔH _† = 16.0 ± 0.6 kcal mol ^-1, and ΔS _† = 12.8 ± 2.3 e.u. Intramolecular exchange in [(R ₃P) ₃Rh(X)] occurs (DFT, MP2//BP86) via a distorted trigonal transition state (TS) with X in an axial position trans to a vacant site. The rearrangement is governed by a combination of steric and electronic factors and is facilitated by bulkier ligands on Rh as well as by strongly donating X that stabilize the TS. The Rh atom in [(H ₃P) ₃Rh(X)] has been shown to be more negatively charged (NPA) for X = CF ₃ than for X = CH ₃, despite the strongly oppositely charged carbon atoms of the CF ₃ (+0.79e) and CH ₃ (-0.96e) ligands. Clarification of stereochemical rigidity (X = halide, CN, OR, NR ₂) versus fluxionality (X = H, Alk, Ar, CF ₃) is provided, along with a resolution of the long-standing contradiction between the electronwithdrawing effect of CF ₃ in organic compounds and its strong trans influence (electron donation) in metal complexes.