“Almost Delocalized” Intervalence Compounds

Interest is high in +M-bridge-M systems that have very low barriers to intramolecular electron transfer giving M-bridge-M+ ("almost delocalized" systems). Hush showed how to evaluate the electronic coupling across the bridge (H) from the M-to-M charge-transfer optical absorption band, but did not point out that his classical model causes the extinction coefficient to suddenly drop to zero at a photon energy of 2 H. Ignoring this band cutoff leads to a low estimation of H.