Structure and fluxional behaviour of some aqua-complexes of rhodium(III) and iridium(III)

Abstract

The single-crystal X-ray structure of trans,mer-[IrCl₂(H₂O)(PMe₂Ph)₃][ClO₄] confirms a structure based on ¹H and ³¹P-{¹H} n.m.r. spectroscopy. The weakly bonded H₂O ligand is trans to PMe₂Ph, a ligand with a strong trans influence, and has a long Ir–O bond [2.1 89(6)Å] and a short Ir–P bond [2.249(3)Å]trans to it. The phosphine ligand site exchange, apparent in the n.m.r. spectra of this compound and its rhodium analogue, is a result of rate-determining H₂O dissociation to give the five-co-ordinate intermediates [MCl₂(PMe₂Ph)₃]⁺(M = Rh or Ir), which are probably squarepyramidal species undergoing rapid pseudo-rotation. The rates of PMe₂Ph exchange and the rates of exchange of free and co-ordinated H₂O are similar in each case but greater for Rh than Ir.