[(Ph3P)(3)Rh(F)] reacts with CF3SiMe3 to produce trans-[(Ph3P)(2)Rh(CF2)(F)] (1; X-ray), which is equilibrated with a number of species in solution. Addition of excess Ph3P shifts all of the equitibria to [(Ph3P)(3)Rh(CF3)] (2; X-ray) as the only NMR-observable and isotable (84%) species. Complex 2 is uniquely highly fluxional in solution, maintaining ligand exchange even at -100 degrees C (12.1 s(-1)). Activation parameters have been determined (variable-temperature P-31 NMR) for the similar but slower exchange in the Me analogue of 2, [(Ph3P)(3)Rh(CH3)]: E-a = 16.5 +/- 0.6 kcal mol(-1), Delta G(not equal) 12.9 kcal mol(-1) (calculated at -30 degrees C), Delta H-not equal 16.0 +/- 0.6 kcal mol(-1), and Delta S-not equal 12.8 +/- 2.3 e.u. Intramolecular exchange in [(R3P)(3)Rh(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)(3)Rh(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)