PARAMETER SCALE FOR SUBSTITUENT EFFECTS IN CYCLOPENTADIENYL COMPLEXES BASED ON GAS-PHASE ELECTRON-TRANSFER EQUILIBRIUM STUDIES OF RUTHENOCENE DERIVATIVES

Gas-phase electron-transfer equilibrium (ETE) studies have been used to obtain the free energies of ionization at 350 K for a number of ruthenocene derivatives of general formula LL'Ru, where L and L' are derivatives of the eta5-cyclopentadienyl ligand (Cp). Equilibrium constants were determined by using Fourier transform ion cyclotron resonance mass spectrometry Various metallocene and organic reference compounds have been used to establish the free energies of ionization (DELTAG(i)-degrees), and the derived values cover a range from approximately 135 to 195 kcal mol-1. A parameter scale (gamma) for Cp substituent effects is derived from the data by assigning anchor parameters to Cp and eta5-pentamethylcyclopentadienyl (Cp*). The ETE results suggest that L = fluorenyl does not stabilize the oxidation of Cp*RuL to the extent that L = Cp* does, with the effect of L = Flu being approximately 65% of the Cp* effect. The consistency of the calculated free energies of ionization based on the assumption of ligand additivity is within approximately 3 kcal mol-1. The potential usefulness of the gamma parameters in correlation and prediction of Cp substituent effects in organometallic reactivity and catalysis is considered.