Intervalence‐Transfer Absorption. Part 2. Theoretical Considerations and Spectroscopic Data

Towards a comprehensive model for the electronic and vibrational structure of the Creutz–Taube ion

“…The resulting electron hole may thus be localized on one of the metal atoms, producing a symmetry-broken charge-localized mixed-valence state in which one metal is Ru(II) and the other Ru(III), or it may be delocalized over both the metals producing non-integral valencies. At the time of its design, nearly 40 years ago, charge-localized mixed-valence systems had recently been characterized for the first time (Robin 1962;Day 1963;Allen & Hush 1967;Hush 1967) but delocalized complex ions and intermediate species had not yet been discovered. Indeed, even to the present day, understanding of the location and motion of electrons remains a significant conceptual challenge as the BornOppenheimer adiabatic approximation (Born & Oppenheimer 1927), which forms the basis for almost all chemical understanding, applies only to each of these extreme limits, whereas mixed-valence compounds may in reality adopt any mixture of the two.…”

“…Basically, the motion of the electron hole becomes coupled to the nuclear motions in a process known as vibronic coupling (Fischer 1984). While chemical intuition may be readily applied to estimate and interpret the properties of systems in either of the standard adiabatic limits, for example leading to both the Robin-Day qualitative classification scheme for mixed-valence compounds based on X-ray crystallography (Robin & Day 1967) and the Hush method for quantitative analysis of optical spectra (Hush 1967), a simple means of interpreting systems in the intermediate non-adiabatic region is not available.…”

Probing the localized-to-delocalized transition

“…The underlying theoretical principles describing the localized-to-delocalized transition between class II and III were developed by Hush in a semiclassical treatment with further elaboration by Sutin and co-workers (Hush 1967(Hush , 1968Creutz et al 1994;Brunschwig & Sutin 1999;Brunschwig et al 2002). Piepho, Krausz and Schatz (PKS) developed a quantum treatment with coupled electron and nuclear coordinates resulting in energies and wave functions dependent on both (Piepho et al 1978;Schatz 1980;Wong & Schatz 1981).…”

Mixed valence: origins and developments

“…The molecular orbital theory of the corresponding IT optical transition which, like that of the thermal transition, can be formulated in terms of linear electronphonon coupling, was also given by Hush (1967). According to this theory, the envelope of the intervalence electronic absorption band yields three parameters defining the dynamics of thermal transfer as well as that of the corresponding optical transfer.…”