Fluxionality of [(Ph3P)3Rh(X)]: The Extreme Case of X = CF3

[(Ph3P)3Rh(F)] reacts with CF3SiMe3 to produce trans-[(Ph3P)2Rh(CF2)(F)] (1; X-ray), which is equilibrated with a number of species in solution. Addition of excess Ph3P shifts all of the equilibria to [(Ph3P)3Rh(CF3)] (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, [(Ph3P)3Rh(CH3)]: 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 [(R3P)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 [(H3P)3Rh(X)] has been shown to be more negatively charged (NPA) for X = CF3 than for X = CH3, despite the strongly oppositely charged carbon atoms of the CF3 (+0.79e) and CH3 (−0.96e) ligands. Clarification of stereochemical rigidity (X = halide, CN, OR, NR2) versus fluxionality (X = H, Alk, Ar, CF3) is provided, along with a resolution of the long-standing contradiction between the electron-withdrawing effect of CF3 in organic compounds and its strong trans influence (electron donation) in metal complexes.