Intervalence charge transfer

In spectrometry, the intervalence charge transfer, sometimes called the "IVCT" or even "IT," is an absorption in the electromagnetic spectrum that is characteristic of bi- and polymetallic coordination complexes that are mixed-valence. According to IUPAC's Goldbook, the IVCT is an "electron transfer (thermal or photoinduced) between two metal sites differing only in oxidation state. Quite often such electron transfer reverses the oxidation states of the sites." [http://goldbook.iupac.org/I03125.htmlArticle: intervalence charge transfer] Inter valence charge transfer is also known as valence tautomerization or metal to metal charge transfer (MMCT). The IVCT band is usually observed in the visible or near infrared region and is broad. The process can be described as follows::L_nM⁺-bridge-M'L_n + hν → L_nM-bridge-M'⁺L_nwhere L is a ligand.

Mixed valency and the IT band

Since the energy states of valence tautomers affect the IVCT band, the strength of electronic interaction between the sites, known as α (mixing coefficient), can be determined by analysis of the IVCT band. [G. L. Miessler and D. A. Tarr “Inorganic Chemistry” 3rd Ed, Pearson/Prentice Hall publisher, ISBN 0-13-035471-6.] Depending on the value of α, the mixed valence complexes are classified into three groups:
*class I: α ~ 0, the complex has no interaction between redox sites. No IVCT band is observed.
*class II: 0 < α < 0.707, intermediate interaction between sites. An IVCT band is observed.
*class III: α > 0.707, interaction between redox sites is very strong. It is better to consider these sites as one united site, not as two isolated sites.

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