. Can. J. Chem. 68,488 (1990). Activation parameters for axial-equatorial fluorine exchange in arylfluorophosphoranes RC6H4PF3H, where R = o-CF3, m-CF3, p-CF3, m-CH3, were studied by the dynamic nmr technique: hGZg8 varied between 53 and 56 kl mol-'.The synthesis of difluoromethoxyphenylphosphorane, PhPF2(H)OMe, from PhPF2 and MeOH is catalyzed by small amounts of Et3N, pyridine, PhPF3H, or HF. Rapid intermolecular ligand exchange occurs in PhPF2(H)OMe after addition of methanol and a base such as triethylamine or pyridine. Under these conditions, exchange of fluorine, hydrogen, and methoxy ligands occurs, as shown by IH, "F, and 3 1~ nmr. From a line shape analysis of the 3 1~{ 1~} nrnr spectrum, the rate of P-F cleavage was found to be first order (1.17 2 0.2) in Et3N concentration, with = 50 kl mol-I and AS" = -67 J mol-' degP'. An equilibrium constant of 1.8 at 25°C was found for the reaction of PhPF2(H)OMe with PhPF2. Key words: axial-equatorial fluorine exchange in RC6H4PF3H intermolecular exchange in the PhPF2(H)OMe-MeOH-base system; 31P and 19F nmr study of ligand exchange in phosphoranes. de la forme des bandes du spectre rmn du 3 1~{ 1~} , on a trouvt que la vitesse de clivage de la liaison P-F est du premier ordre(1,17 * 0,2) en Et3N alors que le AGg8 = 50kl mol-I et le AS" = -67 J mol-I deg-'.