Gas-phase reactions of chromium and chromium fluoride cations CrFn+ (n = 0-4) with phosphane

Ligand and Substrate Effects in Gas-Phase Reactions of NiX+/RH Couples (X=F, Cl, Br, I; R=CH3, C2H5,nC3H7,nC4H9)

The reactions of small saturated hydrocarbons by gaseous nickel cations NiX+ (X=F, Cl, Br, I) are investigated by means of electrospray ionization mass spectrometry. The halide cations are obtained from solutions of the corresponding Ni(II) salts in water or methanol as solvents. NiF+ is the only Ni(II) halide complex that brings about thermal activation of methane. The branching ratios of the observed reactions with C2H6, C3H8, and nC4H10 are shifted systematically by changing the nature of both the ligand X and the substrate RH. In the elimination of HX (X=F, Cl, Br, I), the formal oxidation state of the metal ion appears to be conserved, and the importance of this reaction channel decreases in going from NiF+ to NiI+. A reversed trend is observed in the losses of small closed-shell neutral molecules, that is, H2, CH4 and C2H6, which dominate the gas-phase ion chemistry of NiI+/RH couples. Additionally, inner-sphere electron-transfer reactions take place for a few systems, that is, the delivery of hydride or methanide ions from the hydrocarbon to NiX+ in the course of which the hydrocarbon is converted to a carbenium ion and the cationic metal complex gives rise to a neutral RNiX molecule (R=H, CH3). This process gains importance with decreasing atomic number of the halides and with increasing the size of the alkane. Thus, it constitutes the major pathway in the reactions of NiF+ with propane and n-butane, whereas it is not observed for any of the NiI+/RH couples investigated. Concerning the regioselectivity of the reactions with propane and n-butane, heterolytic cleavage of secondary carbon--hydrogen bonds is clearly preferred compared to that of primary ones, as revealed by deuterium labeling studies. For the NiF+/C3H8 couple, the selectivity of the hydride transfer is as large as 360 in favor of the secondary positions. Though smaller, large preferences for the activation of secondary C--H bonds are also operative in homolytic bond activation of RH (R=nC3H7, nC4H9).

Gas-phase ion chemistry in organometallic systems

This review essentially deals with positive ion/molecule reactions occurring in gas-phase organometallic systems, and encompasses a period of time of approximately 7 years, going from 1997 to early 2004. Following the example of the excellent review by Eller & Schwarz (1991; Chem Rev 91:1121-1177), in the first part, results of reaction of naked ions are presented by grouping them according to the neutral substrate, while in the second part, ligated ions are grouped according to the different ligands. Whenever possible, comparison among similar studies is attempted, and general trends of reactivities are evidenced.

Coordination and dehydrogenation of PH(3) by 23 transition metal ions in the gas phase: FTICR experiments and density functional interpretations

The reactions of 23 transition metal ions M+(g) with phosphane (PH(3)) have been investigated using Fourier transform ion cyclotron resonance spectrometry. Two main reaction pathways are observed, sequential dehydrogenation of multiple (up to nine) phosphane molecules, and addition of multiple (up to four) phosphane molecules. The addition and dehydrogenation reaction pathways are, for the most part, mutually exclusive for a particular metal. Dehydrogenation of phosphane is observed for metal ions toward the lower left of the transition block, and the addition process is predominant toward the top right. The dehydrogenation process is considerably faster than the addition, and sequentially produces the species MPH+, MP(2)(+), MP(3)H(+), MP(4)(+), etc. Density functional calculations were used to evaluate collision trajectories, reaction mechanisms, structures of intermediates (including a number of MoP(x)H(y)(+) species), and to identify the factors determining whether each metal reacts by dehydrogenation or addition. Insertion of M+ into P-H bonds is calculated to be a barrierless, exergonic process for only those metals which are observed to dehydrogenate phosphane molecules. A reaction mechanism involving the transfer of H atoms from P to M followed by the elimination of H(2) from M is profiled for Ru. The energetic origins of the metal influence on reaction type have been traced to the energy changes for the first stage migration of H from P to M. These experimental and theoretical results should be valuable in applications where PH(3(g)) is used in the generation of bulk and surface materials involving metal phosphides.