Kinetics of ligand exchange in rhodium-(I) and -(III) complexes from magnetisation-transfer measurements; the crystal structure of [Rh(NH3)(PPh3)3]ClO4

Abstract

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 PPh₃trans to PPh₃ and PPh₃trans 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 complex. 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.