Olefin rotation in platinum(II) olefin complexes. Part IV. Energies of activities for rotation of substituted olefins in square-planar PtCl2(L)(olefin) and PtCl(acac)(olefin) complexes

Abstract

The energy barriers to rotation of the olefin in the complexes cis-PtCl₂L(olefin) and PtCl(acac)(olefin) have been measured by variable-temperature n.m.r. spectroscopic techniques (L = tertiary arsine or phosphine: olefin = a series of monosubstituted, 1, 1-disubstituted, cis- and trans-1, 2-disubstituted, and trisubstituted ethylenes). The measured ΔG^‡_Tc values are dependent on both the bulk and electronegativity of the substituents and on the symmetry of the olefin. The orientation of the olefin in the ground state, as deduced from a study of platinum–proton coupling constants, is strongly influenced by steric factors. There is evidence for a rotational distortion of the co-ordinated olefin about the carbon–carbon axis in the complex PtCl(acac)(Bu^tCHCH₂).