Ligation kinetics as a probe for relativistic effects in ion chemistry: Gas-phase ligation of late atomic transition metal cations with OCS and CH3Cl at room temperature

Abstract

The kinetics of ligation with OCS and CH3Cl were measured to assess the role of relativistic effects in these chemical reactions with the three d10s1 transition metal cations Zn+, Cd+ and Hg+ as well as the three d10 coinage cations Cu+, Ag+ and Au+ and the three d9 cations Ni+, Pd+ and Pt+. Measurements were performed at 295K in helium bath gas at 0.35Torr using an Inductively-Coupled Plasma/Selected-Ion Flow Tube (ICP/SIFT) tandem mass spectrometer. Rate coefficients are reported for OCS and CH3Cl ligation of Zn+, Cd+ and Hg+ and for OCS ligation with the adjacent d10 coinage metal cations. A strong rate enhancement is observed for Hg+ and Au+, similar to that we have reported recently for Pt+ and Au+ reacting with other ligands. We attribute this rate enhancement to the enhancement in the Hg+-OCS, Au+-OCS and Hg+-CH3Cl binding energies expected from relativistic effects. Comparisons are made with the measured rates of ligation of the adjacent main group d10s2 atomic metal cations Ga+, In+ and Tl+ with OCS and CH3Cl and which don’t exhibit a relativistic effect. Further comparisons are made with the rates of ligation of Zn+, Cd+ and Hg+ with CH4, CH3F, and D2O that we reported previously and these also exhibit relativistic effects with Hg+. The relativistic effect is obfuscated by electron transfer to Hg+ with NO, NO2, CS2, NH3, C6H6, C6F6 and C5H5N as we measured previously. Ligation with O2, CO, CO2, N2O and SF6, is immeasurably slow, too slow to exhibit a relativistic effect with Hg+ under our experimental conditions. Furthermore, the additional new measurements with OCS and CH3Cl provide examples of the occurrence of bimolecular channels in competition with ligation, such as S atom transfer with OCS and H2 or HCl elimination with CH3Cl. Finally, we compare the maximum computed relativistic contraction of the 6s shell reported for Au with the occurrence of a maximum in the rate coefficient of ligation for the third-row transition metal cations Pt+, Au+ and Hg+.