Ferrocene Acidity and C-H Bond Dissociation Energy via Experiment and Theory

The gas-phase acidity of ferrocene (ΔH°_acid(1) = 391.5 ± 1.3 kcal mol^-1) and electron affinity of the ferrocenyl radical (EA(1r) = 1.74 ± 0.08 eV) were measured in a Fourier transform mass spectrometer and combined in a thermodynamic cycle with the known ionization energy of the hydrogen atom to afford the C-H bond dissociation energy of 1 (BDE(1) = 118.0 ± 2.3 kcal mol^-1). Companion M06-2X but not B3LYP computations reproduce each of these thermodynamic quantities and are in accord with an unusually strong aromatic C-H BDE. Natural population analysis and atomic polar tensor charges indicate that a covalent description of ferrocene with a neutral iron atom is a better representation of this compound than an ionic one with a doubly charged Fe²⁺ center. Predicted structural differences upon deprotonation of MCp₂, M = Fe and Mg, are also in accord with this view.