Optical transitions of symmetrical mixed-valence systems in the Class II–III transition regimeElectronic supplementary information (ESI) is available: derivation of eqn. (39c), table summarizing the relationships between band maxima and band widths predicted by the two-state model and table of spectral properties of mixed-valence ruthenium(II)/(III) bridged by pyrazine and dicyanamide. See http://www.rsc.org/suppdata/cs/b0/b008034i/