Gas-phase coordination of Mg+, (c-C5H5)Mg+, and (c-C5H5)2Mg+ with saturated hydrocarbons

The coordination of the electronic ground states of Mg+, (c-C5H5)Mg+, and (C-C5H5)(2)Mg+ With the straight-chain saturated hydrocarbons methane, ethane, n-propane, n-butane, II-pentane, n-hexane, and n-heptane has been investigated in the gas phase in helium 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. Only single ligation was observed. The rate of ligation was found to depend on the size of the saturated hydrocarbon, increasing with increasing size until the collision rate is reached, and on the degree of ligation with c-C5H5. Mg+ was found to be unreactive with methane and ethane, k < 10(-13) cm(3) molecule(-1) s(-1), but ligation was observed with the other hydrocarbons, k greater than or equal to 7 x 10(-12) cm(3) molecule(-1) s(-1). Single ligation of Mg+ with C-C5H5 substantially enhances the efficiency of subsequent ligation: the ligation is rapid, k greater than or equal to 1.4 x 10(-10) cm(3) molecule(-1) s(-1), with all the saturated hydrocarbons investigated. Double ligation with c-C5H5 completely shuts down the efficiency of ligation: no reaction was observed between the saturated hydrocarbons and the full-sandwich magnesocene cation, k < 10(-13) cm(3) molecule(-1) s(-1). A linear correlation is reported between the measured thresholds for multicollision-induced dissociation and the polarizability of the hydrocarbon ligand. With a view to published calculations by Bauschlicher et al. for Mg+-CH4 and Mg+-C2H4, this correlation is consistent with end-on structures for the Mg+-L and (c-C5H5)Mg+-L species observed in this study. A detailed study using density functional theory for the interaction of Mg+ with three rotamers of n-pentane indicates that interconversion of different isomers of Mg+-n-C5H12 should be easy, even at room temperature and that the ligated species Mg+-n-C5H12 is dynamic rather than static. Stronger bonding (by a factor of about two) is indicated for (c-C5H5)Mg+-L by the multicollision CID experiments, and this is consistent with a formal description of (c-C5H5)Mg+ as (c-C5H5)Mg--(2+).