KINETICS OF LIGAND-EXCHANGE IN RHODIUM-(I) AND RHODIUM-(III) COMPLEXES FROM MAGNETIZATION-TRANSFER MEASUREMENTS - THE CRYSTAL-STRUCTURE OF [RH(NH3)(PPH3)3]CLO4

Results from magnetisation-transfer measurements on [RhCl(PPh3)3] and [RhH2Cl(PPh3)3] have been compared with data for analogous complexes obtained by replacing Cl with ONO2 and NH3. For the chloro- and ammine-rhodium(I) complexes the rate constants for phosphine isomerisation and overall phosphine dissociation are similar and much lower than those observed for the nitrato complex. Nitrogen-15 NMR measurements show that this difference is probably due to the greater substitution lability of ONO2 over Cl and NH3. For the rhodium(III) complexes it was possible to obtain rate constants for phosphine dissociation for both PPh3 trans to PPh3 and PPh3 trans to H and similar rate constants are found for [RhH2Cl(PPh3)3] and [RhH2(ONO2)(PPh3)3]. These are larger than those for [RhH2(NH3)(PPh3)3]ClO4, and this is consistent with going from a neutral to a cationic compared in all three cases the rate constant for the fastest phosphine dissociation (trans to H) mirrors the rate of hydride isomerisation. The crystal structure of [Rh(NH3)(PPh3)3]ClO4 has been determined and the structural data are compared with those of other square-planar rhodium(I) complexes.