Sequential ligation of Mg+, Fe+, (c-C5H5)Mg+, and (c-C5H5)Fe+ with ammoniain the gas phase: Transition from coordination to solvation in the sequential ligation of Mg+

Experimental results are reported that track the kinetics of the sequential ligation of Mg+, Fe+, (c-C5H5)Mg+, and (c-C5H5)Fe+ with ammonia in the gas phase as a function of the number of ligands added. The energetics of the sequential ligation of Mg+ with ammonia has also been followed theoreticall y. Molecular orbital calculations with density functional theory (DFT) perf ormed at the B3LYP/6-31+G(d) level have been used to compute the binding en ergies for direct coordination of the ammonia molecules with Mg+ and for so lvation that involves N-H ... N interactions. Single-point calculations wer e also done using the optimized geometries from B3LYP/6-31+G(d) at B3LYP/6- 311++G(2df,p) and MP4SDTQ(fc)/G-311++G(2df,p). Relative binding energies an d standard enthalpies of formation (Delta H degrees(f,298)) have been calcu lated at all three levels of theory investigated. The experiments were perf ormed with a selected-ion flow tube (SIFT) apparatus in helium buffer gas a t 0.35 +/- 0.01 Torr and 294 +/- 3 K. The measured rate coefficients for li gation of both atomic metal ions exhibit a maximum for the second addition of ammonia, which disappears in the presence of a c-C5H5 substituent as the rate of Ligation of Mg+ and Fe+ is enhanced by about 2 orders of magnitude . These trends have been interpreted in terms of the dependence of the Life time of the collision intermediate on its degrees of freedom and the depth of its potential energy well. For the ligation of Mg+, we propose that the precipitous drop in the observed rate of ligation of the fourth Ligand and the emergence of a weakly bonded population of ligated ions in measured mul ticollision-induced dissociation spectra for Mg(NH3)(3)(+) and Mg(NH3)(4)() may signify a change in the nature of the bonding from direct bonding to hydrogen bonding in a second coordination shell with the addition of the th ird ligand. The observed variations in onset energy for multicollision-indu ced dissociation generally are consistent with a negative trend in the liga tion energy with increasing ligation for the four ligated systems investiga ted.