Electronic Coupling and Electron Transfer in Mixed-Valence Systems with Covalently Bonded Dimetal Units

Formation of metal–metal (MM) covalent bonds breaks the degeneracy of the d orbitals of a transition metal atom and renders the dimetal unit (M₂) a unique electron configuration, for example, σ²π⁴δ² for quadruply bonded M₂ ²ⁿ⁺, in which only the δ electrons are in the valence shell and redox active. This makes them desirable mixed-valence (MV) compounds for study of electronic coupling (EC) and electron transfer (ET). This chapter discusses synthetic strategies for the dimers of dimers, mixed valency, and ET kinetics and dynamics of various M₂–bridge–M₂ MV systems. Factors that govern the degree of coupling and ET kinetics, including the nature of the metal ion (Mo vs. W) and chelating atoms (N vs. O vs. S) as well as the length, conjugation, and conformation of the bridge, have been investigated. Optical analysis and study on the MV compounds in different Robin–Day’s regimes (Class I, Class II, and Class III) provide verification and refinement to the mixed-valence theories. The MV systems that assemble two Mo₂ units through hydrogen bonds have been exploited to elucidate the efficiency and mechanisms of ET across hydrogen-bonded interfaces. In addition, studies of axially linked diruthenium (Ru₂) dimers in terms of EC are also presented in this chapter.