Electron-, anion-, and proton-transfer processes associated with the redox chemistry of Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄) and its protonated form [Fe(η⁵-C₅Ph₅)((η⁶-C₆H ₅)C₅Ph₄H)]BF₄

Voltammograms of microcrystals of Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄) and [Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄H)]BF₄ mechanically attached to graphite or gold electrodes are well-defined when the electrode is placed in (70:30) water/acetonitrile (0.1 M electrolyte) media. The simplest processes at the electrode-solvent (electrolyte) interface are the chemically reversible oxidation of Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄), Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄)_(solid) + X^-_(solution) ⇌ [Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄)][X]_(solid) + e^- when X^- is the electrolyte anion (ClO₄^-, BF₄^-, Cl , or F^-), and the chemically reversible reduction of [Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄H)]BF₄, [Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄H)][BF₄] _(solid) + e^- ⇌ Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄H)_(solid) + BF₄^-_(solution), when BF₄^- is the electrolyte anion. Anion exchange between BF₄^-_(solid) in [Fe-(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄H)]BF₄ and the electrolyte anion, X^-_(solution), is rapid so that the potentials of both processes are dependent on the electrolyte anion. Cyclic voltammograms scanned over a potential range encompassing both processes show that interconversion of [Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄H)]⁺ and Fe(η⁵-C₆Ph₅)((η⁶-C ₆H₅)C₅Ph₄) occurs when either [Fe-(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄H)]⁺ or Fe(η⁵-C₅Ph₅)((η₆-C ₆H₅)C₅Ph₄) is initially present on the electrode surface until ultimately a voltammogram containing responses for both processes is achieved for a given electrolyte. Furthermore, the relative proportion of the two processes is a function of the "pH" of the solution phase, implying that the interfacial reaction Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄)_(solid) + H⁺_(solution) + X^-_(solution) ⇌ [Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄H)][X]_(solid) is chemically reversible. While interconversion of Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄) and [Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄H)]⁺ is slow, electrochemical oxidation of Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅-Ph₄) _(solid) to [Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄)][X]_2(solid) leads to very rapid formation of [Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄H)][X]_(solid), possibly via the reaction scheme Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄)_(solid) + X^-_(solution) ⇌ [Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄)][X]_(solid) C₆H₅)C₅Ph₄)][X]_(solid) + X^-_(solution) ⇌ [Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄)][X]_2(Solid) + e^-; 2[Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄)][X]_2(solid) + 2H₂O → 2[Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄H)][X]_(solid) + O_2(solution) + 2H⁺_(Solution) + 2X^-_(solution). However, the possible involvement of radical-based pathways in the conversion of [Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄)][X]_2(solid) to [Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄H)]-[X]_(solid) cannot be excluded. An additional process, which is believed to be ligand based, is observed at a very positive potential. Electrospray mass spectrometric data confirm that [Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄)]⁺ is a product of oxidation of the parent compound and that conversion of both Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄)_(solid) and its cation to [Fe(η⁵-C₅Ph₅)((η⁶-C ₆H₅)C₅Ph₄H)]X_(solid) occurs, while the electrochemical quartz crystal microbalance data verify that anion transport across the electrode-solid-solvent (electrolyte) interface accompanies the electron- and proton-transfer reactions, thereby achieving charge neutralization.