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 Cp2M n, CpFe-2, Cp*Ni-2, Cp*2Os, Cp2Cr, and Cp2Co (Cp = eta5-cyclopentadie nyl, Cp = eta5-pentamethylcyclopentadienyl) have been determined by u sing the electron-transfer equilibrium (ETE) technique and Fourier tra nsform ion cyclotron resonance mass spectrometry. The high-resolution valence photoelectron spectra of bis(benzene)chromium(0), Bz2Cr, Cp2O s, and Cp2Ru have also been measured. Most of the DELTAG(i)DEGREES va lues 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 as signed 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 b etween the integrated heat capacities for Bz2Cr and Bz2Cr+ is also ass umed 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, es timates are given for the average heterolytic and homolytic M-Cp bond disruption enthalpies of Cp2Cr+/0 and Cp2Co+/0. Cyclic voltammetry exp eriments (CH3CN/0.1 M Bu4NPF6) for the decamethylmetallocenes, includi ng Cp2Ru, were performed in order to determine differential solvation energies, DELTADELTAG(solv)degrees, for the +/0 redox couples. Genera lly, DELTADELTAG(solv)degrees values for the decamethyl derivatives ar e in the range -21 to -29 (+/-4) kcal mol-1. Electron-transfer kinetic s for several metallocene couples were measured from the approach to e quilibrium in the ETE experiments, and couples that involved Cp2M com pounds were observed to have rate constants less than 10% of the Lange vin collision frequency when the free energy change was in the range 0 to -6 kcal mol-1.