Equilibrium Isotope Effects in Cationic Transition-Metal(I) Ethene Complexes M(C₂X₄)⁺ with M = Cu, Ag, Au and X = H, D

Abstract

Mass spectrometric methods are used to examine the H/D equilibrium isotope effects (EIEs) of cationic M(C 2 X 4 ) + complexes (X = H, D) of the coinage metals M = Cu, Ag, and Au. Different experimental methods to determine the EIEs are applied, and the best precision is achieved for measurements involving independently prepared mixtures of ethene and [D 4 ]-ethene. In addition, ab initio calculated frequencies are used to derive the corresponding EIEs. In the evaluation of the experimental data as well of the theoretical predictions great care is taken to avoid all possible abberrations and to explicitly treat systematic errors. The measured EIEs of 0.86 ± 0.02 for Cu(C 2 H 4 ) + /Cu(C 2 D 4 ) +, 0.83 ± 0.02 for Ag(C 2 H 4 ) + /Ag(C 2 D 4 ) +, and 0.74 ± 0.02 for Au(C 2 H 4 ) + /Au(C 2 D 4 ) + in favor of the respective deuterated metal complexes are in complete harmony with computed values of 0.86, 0.83, and 0.74, respectively. Interestingly, for the olefin complexes studied here, the EIEs associated with H/D substitution of ethene are not an appropriate measure for the extent of pyramidalization of the methylene groups of the olefin upon complexation to M + .