Gas-Phase Ionization Energetics, Electron-Transfer Kinetics, and Ion Solvation Thermochemistry of Decamethylmetallocenes, Chromocene, and Cobaltocene

The gas-phase free energies of ionization, ΔG_i°, for Cp*₂Mn, Cp*₂Fe, Cp*₂Ni, Cp*₂Os, Cp₂Cr, and CP₂Co (Cp = η⁵-cyclopentadienyl, Cp* = η⁵-pentamethylcyclopentadienyl) have been determined by using the electron-transfer equilibrium (ETE) technique and Fourier transform ion cyclotron resonance mass spectrometry. The high-resolution valence photoelectron spectra of bis(benzene)chromium(0), Bz₂Cr, Cp*₂Os, and Cp*₂Ru have also been measured. Most of the ΔG_i° values are referenced to the estimated ΔG_i° value of Bz₂Cr, for which the narrow first ionization band at 5.473 ± 0.005 eV is assigned as the adiabatic ionization potential. The ΔS_i ° for ionization of Bz₂Cr is assumed to be equal to the electronic entropy change, ΔS_elec ° (=1.4 cal mol^-1 K^-1), and the difference between the integrated heat capacities for Bz₂Cr and Bz₂Cr⁺ is also assumed to be negligible near room temperature (ΔH_i,0° ≈ ΔH_i,350°), leading to ΔG_i°(Bz₂Cr) = 125.6 ± 1.0 kcal mol^-1. Through the use of thermochemical cycles, estimates are given for the average heterolytic and homolytic M-Cp bond disruption enthalpies of Cp₂Cr^+/0 and Cp₂Co^+/0. Cyclic voltammetry experiments (CH₃CN/0.1 M Bu₄NPF₆) for the decamethylmetallocenes, including Cp*₂Ru, were performed in order to determine differential solvation energies, ΔΔG_solv°, for the +/0 redox couples. Generally, Δ ΔG_solv° values for the decamethyl derivatives are in the range -21 to -29 (±4) kcal mol^-1. Electron-transfer kinetics for several metallocene couples were measured from the approach to equilibrium in the ETE experiments, and couples that involved Cp*₂M compounds were observed to have rate constants less than 10% of the Langevin collision frequency when the free energy change was in the range 0 to −6 kcal mol^-1.