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Electron-deficient bismuth clusters are classical examples of homopolyatomic ions (a polyatomic ion composed entirely of a single element). They were originally observed in dilute solutions of bismuth metal in molten bismuth chloride. It has since been found that these clusters are present in the solid state (particularly in salts where germanium tetrachloride or tetrachloroaluminate serve as the counteranions) and endow materials with a variety of interesting optical properties.

Structure
Bismuth polycations are held together by sigma bonds, despite the fact that they possess fewer total valence electrons than would seem necessary for the number of sigma bonds. The shapes of these clusters are dictated by Wade's rules, which are based on the treatment of the electronic structure as delocalized molecular orbitals. The bonding can also be described with three-center two-electron bonds in some cases, such as the Bi53+ cluster.

Bismuth clusters have been observed to act as ligands for copper and ruthenium ions. This behavior is possible due to the otherwise fairly inert lone pairs on each of the bismuth that arise primarily from the s-orbitals left out of Bi-Bi bonding.

Optical Properties
The variety of electron-deficient sigma aromatic clusters formed by bismuth gives rise to a wide range of spectroscopic behaviors. Of particular interest are the systems capable of low-energy electronic transitions, as these have demonstrated potential as near infrared light emitters. Such a property makes these species potentially valuable to the field of near-infrared optical tomography, which exploits the near-infrared window in biological tissue.