Cationic transition-metal complexes. Part V. Synthesis, structure, and dynamic behaviour of bis(π-allyl)-rhodium(III) and -iridium(III) cations

Abstract

The synthesis and characterisation of cationic bis-(π-allyl) or -(π-2-methylallyl) complexes [M(all)₂L₂]BF₄(M = Rh or Ir; L = phosphine, phosphites, arsine, pyridines, and MeCN), and of the neutral complexes [M(all)₂(tpl)](M = Rh or Ir; tpl = tropolonate) and [Rh(all)₂(pyc)](pyc = pyridine-2-carboxylate) is described. ¹H N.m.r. spectra of these complexes show that two equivalently bonded π-allyl ligands are asymmetrically bonded to each metal atom. The dynamic behaviour of these complexes in solution has been studied by variable-temperature ¹H n.m.r. spectroscopy. In the case of the cationic complexes it is concluded that a left-to-right exchange process occurs via initial dissociation of a neutral ligand (L) followed by Berry pseudo-rotation of the resultant five-co-ordinate species. The neutral complexes undergo left-to-right exchange and concomitant syn–anti-exchange via rupture of a metal–carbon bond.