Gas-Phase Coordination of Mg+, (c-C5H5)Mg+, and (c-C5H5)2Mg+ with Small Inorganic Ligands

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 103; no. 32; pp. 6373 - 6382
• Main Authors: Milburn, Rebecca K, Baranov, Vladimir, Hopkinson, Alan C, Bohme, Diethard K
• Format: Journal Article
• Language: English
• Published: American Chemical Society 12.08.1999
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp9908505

The coordination of the electronic ground states of Mg+, (c-C5H5)Mg+, and (c-C5H5)2Mg+ with the inorganic molecules H2, NH3, H2O, N2, CO, NO, O2, CO2, N2O, and NO2 has been investigated in the gas phase at room temperature and moderate pressures. Reaction rate coefficients and product distributions were measured with the selected-ion flow tube (SIFT) technique operating at 294 ± 3 K and a helium buffer-gas pressure of 0.35 ± 0.01 Torr. Rate coefficients were measured for all observed ligation steps (or upper limits in the case of nonreactions), and bond connectivities in the coordinated ions were probed with multicollision-induced dissociation. The rates and extent of ligation were found to depend on the degree of ligation with c-C5H5 and the nature and size of the inorganic ligand. Mg+ was found to be unreactive except with ammonia, which was found to add slowly (k = 4 × 10-12 cm3 molecule-1 s-1). Single ligation of Mg+ with c-C5H5 substantially enhances the efficiency of ligation; the initial ligation is rapid (k > 5 × 10-11 cm3 molecule-1 s-1) with all ligands except H2, N2, and O2. Double ligation with c-C5H5 substantially reduces the efficiency of ligation; no ligation was observed with the “full-sandwich” magnesocene cation. Fast bimolecular ligand-switching reactions occur instead, but only with NH3 and H2O, indicating that these two molecules ligate more strongly to (c-C5H5)Mg+ than does c-C5H5 itself. Higher-order ligation was observed between Mg+ and NH3 and between the “half-sandwich” (c-C5H5)Mg+ cation and CO, NO, NH3, H2O, CO2, N2O, and NO2. After a slow initiation step, ammonia added to Mg+ up to four times to generate Mg(NH3)4 +. Higher-order ligation reactions of (c-C5H5)Mg+ with CO, NO, NH3, CO2, N2O, and NO2 proceed with diminishing rates adding up to at most three ligands. H2O is an exception; at least six H2O molecules were observed to add sequentially to (c-C5H5)Mg+. Variations in the observed rate of this addition have been attributed to the completion of an “inner” coordination shell and the onset of hydrogen bonding in an “outer” coordination shell that accounts for the continuing rapid ligation. Higher-order ligands were removed sequentially by multicollision-induced dissociation with He atoms; there was no evidence for intramolecular interligand interactions leading to unimolecular bond redisposition after ligation.