Dimers of Nineteen-Electron Sandwich Compounds: An Electrochemical Study of the Kinetics of Their Formation

(Chemical Equation Presented). Rate constants for the dimerization of FeCp∗C₆H₆, RuCp∗mes, RhCp₂, RhCp∗Cp, and IrCp∗Cp (Cp∗ = pentamethylcyclopentadienyl; mes = 1,3,5-trimethylbenzene; Cp = cyclopentadienyl) have been investigated by cyclic voltammetry. Rates increased in the order FeCp∗C₆H₆ 蠐 RhCp∗Cp ∼ RhCp₂ < RuCp∗mes and IrCp∗Cp. The difference in rates between the Rh compounds and the Ru and Ir species is consistent with the thermodynamic driving forces for dimerization estimated from DFT calculations. However, the sluggish dimerization of FeCp∗C₆H₆ cannot be rationalized with purely thermodynamic considerations and may be attributable to the very different spin-density distribution in this species. On the basis of the activation parameters for dimerization, determined by variable temperature cyclic voltammetry, and those previously determined for dissociation of the corresponding dimer, the free energy of dimerization for the RhCp∗Cp monomer was estimated to be -43 kJ mol^-1, and the effective redox potential for the RhCp∗Cp⁺/ 0.5(RhCp∗Cp)₂ couple was estimated as -1.84 V vs FeCp₂⁺/FeCp₂.