Olefin rotation in platinum(II) olefin complexes. Part III

Abstract

The barriers to rotation of ethylene in complexes PtXYL(C₂H₄)(X = Cl, Br, or CF₃CO₂trans to the olefin; Y = Cl, Br cis to the olefin, L = tertiary phosphines, phosphites, arsines, or a primary amine cis to the olefin) have been measured by n.m.r. spectroscopic techniques. The measured ΔG†;_Tc values span the range 10·5—13·9 kcal mol^–1 and depend on the nature of the ligands. Comparison of the barriers with the i.r. stretching frequency of the terminal CO group in the analogous complexes PtXYL(CO) demonstrates a significant steric contribution to the energy of rotation. There is n.m.r. spectroscopic evidence for twisting of the olefin about the carbon–carbon axis and of a barrier to rotation of the phosphine ligand of 13 kcal mol^–1 in PtCl₂[(2-MeOC₆H₄)₃P](C₂H₄).