The cationic (C2H4)M+ complexes (M = Cu, Ag, and Au) have been examined by different ab initio molecular orbital, density functional (DFT), and density functional/Hartree−Fock (DFT/HF) hybrid methods using relativistic effective core potentials and a quasi-relativistic approach to account for relativistic effects. For (C2H4)Au+ a substantial relativistic stabilization is observed, such that the computed binding energies are almost twice as high than for (C2H4)Ag+ and still significantly higher than for (C2H4)Cu+. Structural features and energetics obtained at the various computational levels, although they differ significantly in their computational demands, are in satisfying agreement with each other, adding to the level of confidence that can be attributed to the computationally economic DFT and DFT/HF hybrid methods. In order to determine the nature of the bonding in these (C2H4)M+ complexes, an energy decomposition scheme is applied to the DFT results. For all three metal cations, the interaction with ethylene shows large covalent contributions. The major part of the covalent terms stems from σ-donor contribution from the ligand to the metal, whereas π-acceptor bonding (back-bonding) is less important. An atoms-in-molecules (AIM) analysis of the charge density distribution reveals cyclic structures for (C2H4)Au+ and (C2H4)Cu+, whereas (C2H4)Ag+ is T-shaped.

The Pt + -mediated coupling of methane and ammonia has been studied both experimentally and computationally. This system serves as a model for the Degussa process for the industrial production of the valuable feedstock hydrogen cyanide. Mass spectrometric studies demonstrate that C-N bond formation is catalyzed efficiently by Pt + . Details of the experimentally observed reaction channels have been explored computationally using the B3LYP hybrid DFT/HF functional. In the first reaction step, Pt + dehydrogenates CH 4 to yield PtCH 2 + ; in contrast, dehydrogenation of ammonia by Pt + is endothermic and does not occur experimentally. Starting from PtCH 2 + and NH 3 , C-N bond formation, which constitutes the crucial step in making HCN from CH 4 and NH 3 , is achieved via two independent pathways. The major pathway is found to be exothermic by 23 kcal mol -1 and yields neutral PtH and CH 2 NH 2 + . The second pathway involves a dehydrogenation to yield the aminocarbene complex PtC(H)NH 2 + (∆ r H ) -36 kcal mol -1 ); dehydrogenation of PtC(H)NH 2 + to PtCNH + is exothermic with respect to PtCH 2 + + NH 3 (∆ r H ) -8 kcal mol -1 ) but hindered by kinetic barriers. A comparison of Pt + with other transition metal cations

The potential-energy surface of the triatomic cation [Cr,O2]+ has been examined by means of ion-cyclotron resonance and sector-field mass spectrometry as well as high-level theoretical methods. Chromium(V) dioxide cation OCrO+ can be generated by electron ionization of CrO2Cl2 and exhibits a doublet ground state. The experimentally determined IE(CrO2) of 9.7 ± 0.2 eV leads to ΔH f(OCrO+) = 209 ± 12 kcal/mol which compares well with a theoretical prediction of 217 kcal/mol. The high-valent chromium(V) dioxide OCrO+ slowly reacts with H2 to form CrO+ and H2O as products. Activation of saturated and unsaturated hydrocarbons, including methane and benzene, is much more efficient and involves C−H as well as C-C bond activation. In the reaction of OCrO+ with CH4 the by-product OCr(OCH2)+ points to the operation of a stepwise mechanism in that initially a single oxo unit in OCrO+ is activated in terms of a [2 + 2] cycloaddition of methane across the Cr−O double bond. A structurally different [Cr,O2]+ cation can be generated by chemical ionization of a mixture of Cr(CO)6 and O2. The experimental findings together with the computations propose that the ions formed consist of a mixture of doublet and quartet states of the dioxide OCrO+ and the dioxygen complex Cr(O2)+ (6A‘‘); in the latter the dioxygen unit is end-on coordinated to the metal. Due to spin conservation, the direct formation of the doublet ground state OCrO+ (2A1) from the ground state reactants Cr+ (6S) and O2 (3Σg -) is not possible; rather, two curve crossings from the sextet via the quartet to the doublet surface in the sequence 6Cr+ + 3O2 → 6Cr(O2)+ → 4OCrO+ → 2OCrO+ are suggested.

Fourier transform ion-cyclotron resonance mass spectrometry is used to investigate the reactivity of mass-selected bimetallic Pt m Au n + clusters (m + n e 4) with respect to the C-N coupling of methane and ammonia. To this end, the reactions of the heterometallic carbene species Pt m Au n CH 2 + with NH 3 as well as those of the bare clusters with CH 3 NH 2 are studied. On the basis of these experiments augmented by deuterium labeling studies, structural assignments for the reaction products are proposed. Surprisingly, only the dinuclear carbene PtAuCH 2 + mediates C-N bond formation to presumably afford the aminocarbene complex PtAuC(H)NH 2 + , whereas the larger bimetallic carbene clusters mainly yield the carbide species Pt m Au n C + ‚NH 3 upon reaction with NH 3 . This difference is rationalized by distinct metal-carbene binding energies. While the pure Au n + clusters do not afford C-N coupling either, they undergo degradation reactions with NH 3 and CH 3 NH 2 that are subject to pronounced even-odd size effects.

The gas-phase reactions of the platinum−carbene clusters Pt n CH2 + (n = 2−5) with the substrates O2, CH4, NH3, and H2O have been investigated by FT-ICR mass spectrometry and compared with previous results for the mononuclear homologue PtCH2 +. The ion−molecule reactions of the clusters Pt n CH2 + with O2 and CH4 are similar to the corresponding processes observed for PtCH2 +. In contrast, a surprising difference evolves in the reactions with NH3 and H2O. Whereas PtCH2 + mediates carbon−heteroatom bond formation as an attractive way toward methane functionalization, the homologous clusters fail in this regard and exclusively yield the carbide complexes Pt n C+·NH3 and Pt n C+·H2O, respectively. The differences in reactivities have been analyzed further by means of kinetic isotope effects, H/D-exchange reactions, and energy-dependent collision-induced dissociations of Pt n CH2 +. Essentially, the higher stabilities of the platinum−carbide clusters Pt n C+ compared to PtC+ cause the change in reactivity. This conclusion is confirmed by the reactions of independently generated clusters Pt n C+ (n = 1−5) with O2, CH4, NH3, and H2O. The results underline the importance of carbide structures in these gas-phase reactions, thereby resembling catalyst deactivation by soot formation on heterogeneous platinum catalysts.

Ion-cyclotron resonance mass-spectrometric experiments are used to examine the reactions of small cationic vanadium clusters V n + and vanadium-oxide clusters V n O m + (n e 5, m e 3) with molecular oxygen. Most of these reactions are very fast (k g 3 × 10 -10 cm 3 molecule -1 s -1 ) and strongly exothermic. Oxidative degradation of the clusters (decreasing n) is generally more favored than oxygen-transfer resulting in V n O m + cations (constant n). Consequently, the mononuclear oxide ions VO + and VO 2 + evolve as the major final products at the end of a reaction cascade. For V 2 + and V 3 + , the complete network of elementary steps is explored by MS/MS experiments. Furthermore, some thermochemical brackets are derived, e.g., for the enthalpies of formation of the ions V 2 O m + (m ) 1-3): 920 kJ/mol e ∆ f H (V 2 O + ) e 1130 kJ/mol; 670 kJ/mol e ∆ f H (V 2 O 2 + ) e 880 kJ/mol, and 420 kJ/mol e ∆ f H (V 2 O 3 + ) e 630 kJ/mol.

The n-propyl/isopropyl isomerization operative in Fe+-mediated dehydrations of n-propyl and isopropyl alcohols has been rigorously analyzed by tandem mass spectrometry and density functional calculations. Metastable ion studies of complexes of atomic Fe+ with a set of selectively deuterated propyl alcohols address structural details of the reversibility of β-hydrogen transfer steps. The labeling distributions in the ionic products reveal that the transiently focused, constitutionally equivalent methyl groups remain distinguishable in the course of n-C3H7 → i-C3H7 isomerizations. This asymmetry causes a memory effect operative in the Fe+-mediated dehydration of n-propyl alcohol. While density functional calculations provide detailed insight into the nature of the reaction intermediates and, inter alia, suggest that the experimentally deduced asymmetry can be attributed to agostic interactions of the iron center with a β-hydrogen atom of the emerging methyl group, a comprehensively consistent, quantitative explanation of the experimentally observed effects cannot be provided by the calculations.

is prepared by reaction of Cr + with monofluoracetone and investigated by means of Fourier transform ion-cyclotron resonance mass spectrometry. CrC 3 H 4 O + only slowly dehydrogenates alkanes larger than propane, but efficiently reacts with alkenes to afford dehydrogenation, ligand loss, and C-C bond cleavage. These reactions can be explained by assuming the metal carbene structure 1 for CrC 3 H 4 O + , which reacts with alkenes by an initial [2+2] cycloaddition. Consecutive cycloreversion results in metathesis products that undergo alkylidene-olefin conversion and elimination of the C-C cleaved fragments. For allene, the corresponding vinylidene-acetylene transformation appears to be particularly favorable. In addition, the experimental findings suggest the occurrence of initial hetero-Diels-Alder reactions that involve the carbonyl group of 1. At elevated energies, such as upon complexation by benzene, 1 can rearrange to form the chromaoxacyclobutene 2 and eliminate propyne.