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Applying the ligand electrochemical parameter approach to sandwich complexes and standardizing to the Fe(III)/Fe(II) couple, we obtained E(L)(L) values for over 200 pi-ligands. Linear correlations exist between formal potential (E degrees ) and the summation operatorE(L)(L) for each metal couple. In this fashion, we report correlation data for many first row transition metal couples. The correlations between the E(L)(L) of the substituted pi-ligand and the Hammett substituent constants (sigma(p)) are also explored.

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The rate constant &R for the highly exothermic, spin-allowed reaction F e O + ( W ) + H2 -Fe+(6D) + H 2 0 is 2 orders of magnitude smaller than the collision rate constant &L (&R = 1.6 X 10-11 cm3 molecule -l s-I; kL = 1.5 X cm3 molecule-' s-l). Labeling experiments using HD and D2 reveal that no strong intermolecular kinetic isotope effects are operative on the rate constant of this reaction. Extremely low reactivities are also observed for the reactions of Coo+ and NiO+ with H2 ( k~ = 1.2 and 2.1 X 10-12 cm3 molecule-l s-l), and there are no kinetic isotope effects within experimental error in the reactions of these metal oxides with D2 as well. The experimental data and theoretical considerations point to a reaction mechanism involving a multicentered transition structure in the rate-determining step. IntroductionThe activation of C-H and C-C bonds of organic molecules by ionic metal oxides, in particular iron oxenoids, is of fundamental interest in organic chemistry as well as biochemistry.] Recently, the C-H bond activation of a series of hydrocarbons? including methane and benzene, by bare FeO+ in the gas phase3 was reported, and mechanistic insight in the elementary steps of these reactions was provided. Here, we describe experiments on the gas-phase reactions of FeO+ with molecular hydrogen. This process, which is analogous to the thoroughly studied reactions of the early transition-metal oxides MO+ (M = Sc, Ti, and V)4 with D2, is, for at least two reasons, of particular importance: (i) In thedetailed study from the Armentrout group4 it is convincingly demonstrated that-in addition to reaction energetics-spin conservation is the prime factor in oxygenation processes. As the ground electronic state of FeO+ corresponds to the W state,5 one should expect that the exothermic, spin-allowed oxidation of molecular hydrogen (eq 1) is a facile process.(ii) If this supposition holds true, one can expect that the simple redox pair described in eq 1 forms the basis for a catalytic cycle (Scheme 1),8 in which molecular hydrogen is oxidized to water by means of FeO+, which itself is formed by reacting Fe+ with convenient oxidants, e.g., N Z O .~~ Although the combustion process H2 + ' / 2 0 2 -H20 is highly exothermic, this reaction is, as is amply known, kinetically hampered.For the purpose of comparison we have also conducted some preliminary experiments with, in terms of electronic structure, the less properly characterized metal oxides MO+ (M = Co, Ni). Experimental SectionThe experiments were performed with a Spectrospin CMS 47X Fourier transform ion-cyclotron-resonance (FTICR) mass spectrometer; the experimental setup has been described in detail elsewhereO9 In brief, the metal ions M+ (M = Fe, Co. Ni) were generated by laser desorption/laser ionization by focusing the beam of a Nd:YAG laser onto a metal target. The cations were extracted from the source and transferred to the analyzer cell by a system of electric potentials and lenses. The isolation of the metal ions most abundant isotope and all subsequent iso...

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Fourier transform ion cyclotron resonance mass spectrometry was used to study ion/molecule reactions of six cationic methyl metallocenes:  [Cp2ZrCH3]+ (1; Cp = η5-cyclopentadienyl), [(Ind)(Cp)ZrCH3]+ (2; Ind = η5-indenyl), [Ind2ZrCH3]+ (3), [ansa-(CH3)2Si(η5-C5H4)2ZrCH3]+ (4), [(Flu)2ZrCH3]+ (5; Flu = η5-fluorenyl), and [ansa-(CH3)2Si(η5-C5Me4)(N-t-C4H9)ZrCH3]+ (6). Rate constants and product distributions for reactions with dihydrogen and various deuterium-labeled and unlabeled alkenes were determined. The electrophilicity of [L2Zr−CH3]+ ions in the gas phase is strongly dependent on the nature of the ancillary ligands L, with electrophilicity decreasing in the order Cp2 > Cp,Ind> Ind2 > Flu2 for reactions with dihydrogen and 1-alkenes. Silyl-bridging of Cp ligands as in 4 appears to increase the electrophilicity somewhat, and the 10-electron ion 6 also has higher electrophilicity than 1. In each reaction used to assess electrophilicity, the probable rate determining transition state involves a 4-center/4-electron species in either C−H or H−H activation. In the reaction of ethylene with [L2Zr−CH3]+, labeling studies show partial-to-complete scrambling of H/D labels before elimination of dihydrogen. Scrambling increases with decreasing electrophilicity of the ion. A model for the intramolecular isotope effect leads to k H/k D = 2.2 for the allylic C−H(D) activation step in reaction of 3 with ethylene. Isotopic labeling has shown that the C−H activation of isobutene by [L2Zr−CH3]+ proceeds via allylic activation only. Dimerization and hydrolysis of [L2Zr−CH3]+ is facile in the gas phase, and the products are analogous to known side reactions in solution chemistry. The polymerization activity of electrophilic zirconium polymerization catalysts does not necessarily correlate with the intrinsic electrophilicity of [L2Zr−CH3]+ species as L is varied. The ion pairing tendency of [L2Zr−CH3]+ in solution is a key contributor to the overall propagation rate, and the stability (K ip) of the ion pair for a given counterion is expected to decrease as L becomes more electron donating and larger. If the ion pair preequilibrium trends dominate the kinetics, then more active catalysts can be obtained by introducing larger ancillary ligands with electron-donating substituents despite the reduced electrophilicity of the metallocenium ion.