Probing relativistic effects in the gas-phase CS2 ligation of late transition metal cations (groups 9–11) with rate measurements and quantum chemical calculations of ligation energies

Abstract

Striking relativistic effects are demonstrated for the ligation of carbon disulphide to late atomic transition metal cations (Groups 9–11) with both measurements of rate coefficients for ligation in helium buffer gas at 0.35 ± 0.01 Torr and 295 ± 2 K and quantum chemical calculations of ligation energies. The rates of ligation with one CS2 molecule were observed to be enhanced by a factor of about 10 going from second row to third-row atomic transition metal cations (Groups 9–11). The addition of a second CS2 ligand, although intrinsically much faster due to effects of degrees of freedom on the lifetime of the intermediate, also exhibits enhancement of the ligation rate for the third-row atomic cations for Groups 10 and 11. The trends in the computed M+-CS2 bond dissociation energies down Groups 9-11 clearly follow the trends in the measured rate coefficients for the first addition of CS2 and, to a degree, the second addition. A novel isodesmic reaction method was used to estimate the percent relativistic contribution to the M+-CS2 bond energy down the periodic table which exceeds 25% for third-row atomic ions. The computations also provided optimized structures for the ligated ions M+(CS2) and M+(CS2)2. A correlation between the measured ligation efficiency and the computed ligation energy provided a measure of the number of vibrational modes active in the stabilization of these ligated ions.