ADIABATIC IONIZATION ENERGIES, BOND DISRUPTION ENTHALPIES, AND SOLVATION FREE-ENERGIES FOR GAS-PHASE METALLOCENES AND METALLOCENIUM IONS

Free energies of ionization for Cp2V, Cp2Mn, Cp2Fe, Cp2Ni, Cp2Ru, Cp2OS (Cp = eta2-cyclopentadienyl), and a series of ferrocene derivatives have been determined through gas-phase electron-transfer equilibrium (ETE) reactions by using Fourier transform ion cyclotron resonance mass spectrometry. Temperature dependence studies involving ETE of ferrocene with N,N-diethyltoluidine lead to a value of the enthalpy of ionization, DELTAH(i)-degrees, for ferrocene of 6.82 +/- 0.08 eV. Experimental and statistical mechanical analyses indicate that the one-electron oxidation of ferrocene is accompanied by a positive entropy of ionization, DELTAS(i)-degrees, most of which is associated with changes in the electronic and vibrational contributions to the partition functions of ferrocene and the ferrocenium ion. Ionization energies of alkylferrocene derivatives are correlated with alkyl Taft parameters. Thermochemical cycles are used to derive estimates of average homolytic and heterolytic bond disruption enthalpies (DELTAH(hom)-degrees and DELTAH(het)-degrees) for selected metallocenium ions. For Cp2M+, the following mean M-Cp bond disruption enthalpies (kcal mol-1) are derived: DELTAH(hom)-degrees (M+-Cp) = 95 +/- 3 (V), 74 +/- 4 (Mn), 91 +/- 3 (Fe), 83 +/- 3 (Ni); DELTAH(het)-degrees (M+3-Cp) = 563 +/- 4 (V), 604 +/- 5 (Mn), 593 +/- 4 (Fe), 659 +/- 4 (Ni). Differential solvation free energies (DELTADELTAG(solv)-degrees) for several metallocene/metallocenium redox couples are derived. With the exception of Cp2V+/0, most of the first transition row metallocene/metallocenium redox couples are estimated to have DELTADELTAG(solv)-degrees values of 38 +/- 5 kcal mol-1, which is consistent with the Born approximation for predicting ion solvation energies. These assessments of bonding and solvation energetics based on gas-phase adiabatic ionization energies lead to a complete thermochemical interpretation of observed solution electrode potentials for the metallocene redox couples studied.