Redox orbital trends

The configuration of the macrocyclic ligand affects the electrochemical properties of Ni(II) complexes (Table I) (56a, 54). For example, the oxidation and reduction potentials of CR,S,R,S)-[Ni(14)]2+ are shifted by +0.14and +0.13 V, respectively, compared with those of the Rfi,S,S isomer. Similar trends are also observed for a series of R,Sfi,S and Rfi,S,S isomers of -methylated cyclam derivatives (61a, 61b). The anodic shift of the redox potentials for the i ,S ,S-Ni(II) complex indicates that the complex is more difficult to oxidize to Ni(III) but easier to reduce to Ni(I), compared with the RJl,S,S complex. This may be related to the reduced ligand field strength of the R,Sfi,S complex, which stabilizes the antibonding -orbitals and thus makes addition of an electron more favorable while removal of an electron is less favorable. 

Redox potential data frequently correlate with parameters obtained by other spectroscopic measurements. The correlation of E° potentials with gas-phase ionization potentials has already been briefly discussed. Electronic transitions observed by UV-visible spectroscopy involve the promotion of an electron from one orbital to another and this can be viewed as an intramolecular redox reaction. If the promotion involves the displacement of an electron from the HOMO to the LUMO, then the redox potentials for the reduction of the compound, °REd, and for its oxidation, °ox, are of importance. For a closely related series of compounds, trends in oxidation and reduction potentials can be related to shifts in the absorption frequency, v. If the structural perturbation causes the HOMO and the LUMO to rise or fall in energy in tandem, then (E°RED — E°ox) will remain constant in such cases the HOMO—LUMO frequency (energy) will be essentially independent of the structural perturbation. Where there is a differential influence of the perturbation on the HOMO and the LUMO, then ( °red E°ox) will vary as will the energy of the electronic transition. In such cases a linear correlation of °red or E°0x may result. In the limit the energy of the HOMO, or more usually the LUMO, will be unaffected by structural perturbation where the acceptor orbital is pinned, direct linear correlation of E°Gx with v should be apparent. With E°ox and v in a common energy unit, the plot E°0x versus v should have a slope close to one.33-36... 

Semiempirical MNDO calculations have been carried out on model pyrylium and thiopyrylium systems (88MI1). The calculated HOMO-LUMO gap in the gas phase correlates well with experimental absorption maxima obtained in solution. Ionization potentials and electron affinities predicted by Koopmans theorem with MNDO orbital energies do not track the observed trends in the experimental redox values. In contrast these are paralleled by the trends predicted by A// values calculated by MNDO and AMI for the open-shell and closed-shell species. 

Contemporary with the proposal of the enthalpy of the redox transition, Matsumoto and coworkers suggested the overlap between the eg orbital (or a band) and the orbital of the hydroxide ion could explain the trends in the oxygen evolution of perovskites [34, 35]. Recently, researchers predicted and verified a volcano relationship with an 6g occupancy close to unity for optimum oxygen evolution activity for perovskites [10]. In addition, they suggested that the covalency of the metal-oxygen bond could serve as a secondary predictor for the catalytic activity. 

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