Inversion of configurations at pyramidal sulphur, selenium, and tellurium in diethyl chalcogen complexes of platinum(II) and palladium(II)

Abstract

The variable-temperature n.m.r. spectra of the complexes trans-[MX₂(ZEt₂)₂](Z = S, Se, or Te; M = Pd or Pt: X = Cl, Br, or I) indicate that inversion of configuration at the pyramidal chalcogen atoms is a facile process. Retention of ³J(¹⁹⁵Pt^–1H) coupling above the coalescence temperatures for the sulphur and selenium complexes, and for cis-[Pt(C₆H₄Me-4)₂(TeEt₂)₂], confirms that the dynamic processes are intramolecular for these complexes at least. The chelate complexes [MX₂(EtSeC₂H₄SeEt)] show related spectroscopic phenomena, which can be interpreted in terms of single inversions at the selenium atoms. The barriers to inversion are in the order Te > Se > S, and are sensitive to the trans influence of the opposite ligands. These barriers are lower than in related pyramidal systems such as sulphoxides or selenoxides, and this may be due to stabilisation of the transition states by the π-acceptor capacity of the metal atoms. The barriers to inversion are less in the palladium complexes than in the analogous platinum derivatives.