The gas-phase free energies of ionization, ΔGi°, 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 ΔGi° values are referenced to the estimated ΔGi° value of Bz2Cr, for which the narrow first ionization band at 5.473 ± 0.005 eV is assigned as the adiabatic ionization potential. The ΔSi ° for ionization of Bz2Cr is assumed to be equal to the electronic entropy change, ΔSelec ° (=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 (ΔHi,0° ≈ ΔHi,350°), leading to ΔGi°(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, ΔΔGsolv°, for the +/0 redox couples. Generally, Δ ΔGsolv° 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry