What is delocalization? The Creutz-Taube ion as Delphic Oracle

INOR 346

Noel S. Hush, hush_n@chem.usyd.edu.au, School of Molecular and Microbial Biosciences, The University of Sydney, Sydney, 2006, Australia
The year 2007 marks the thirty-eighth anniversary of the synthesis of the mixed-valence species (( bis- entanmmineRu)pyrazibe)5+ by Carol Creutz and Henry Taube, now famously known as the Creutz-Taube ion. One motivation of this work was to look for an absorption band 'intervalence band') in the near infra-red region predicted by Hush two years earlier for a symmetrical mixed-valence species, from the properties of which the rate of the corresponding thermal electron transfer process could be calculated in the limit of small electronic coupling between donor and acceptor moieties. Such an absoprtion band was indeed found in this region. In the subsequent years the question of the extent of delocaliztion between the ruthenium centers centering on the proprties of the Creutz-Taube ion has been the subject of much experimental and theoretical concern. This has continued to reveal major ambiguities in our understanding of the important concept of electron delocalization, particularly in solution and other condensed environments. This intriguimg history is briefly outlined, and the present account in the ongoing saga us summarised.