GAS-PHASE IONIZATION ENERGETICS, ELECTRON-TRANSFER KINETICS, AND ION SOLVATION THERMOCHEMISTRY OF DECAMETHYLMETALLOCENES, CHROMOCENE, AND COBALTOCENE

The gas-phase free energies of ionization, DELTAG(i)degrees, for Cp*2Mn, Cp*Fe-2, Cp*Ni-2, Cp*2Os, Cp2Cr, and Cp2Co (Cp = eta5-cyclopentadienyl, Cp* = eta5-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 DELTAG(i)DEGREES values are referenced to the estimated DELTAG(i)degrees value of Bz2Cr, for which the narrow first ionization band at 5.473 +/- 0.005 eV is assigned as the adiabatic ionization potential. The DELTAS(i)degrees for ionization of Bz2Cr is assumed to be equal to the electronic entropy change, DELTAS(elec)degrees (=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 (DELTAH(i,0)degrees almost-equal-to DELTAH(i,350)degrees), leading to DELTAG(i)degrees(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, DELTADELTAG(solv)degrees, for the +/0 redox couples. Generally, DELTADELTAG(solv)degrees 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.