Bismuth polycations are

polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zero. The term molecule may or may not ...

s of the formula . They were originally observed in solutions of bismuth metal in molten

bismuth chloride Bismuth chloride (or butter of bismuth) is an inorganic compound with the chemical formula BiCl3. It is a covalent compound and is the common source of the Bi3+ ion. In the gas phase and in the crystal, the species adopts a pyramidal structure, ...

. It has since been found that these clusters are present in the solid state, particularly in salts where

germanium tetrachloride Germanium tetrachloride is a colourless, fuming liquid with a peculiar, acidic odour. It is used as an intermediate in the production of purified germanium metal. In recent years, GeCl4 usage has increased substantially due to its use as a reagent ...

tetrachloroaluminate Tetrachloroaluminate lCl4sup>− is an anion formed from aluminium and chlorine. The anion has a tetrahedral shape, similar to carbon tetrachloride where carbon is replaced with aluminium. Some tetrachloroaluminates are soluble in organic ...

serve as the counteranions, but also in amorphous phases such as

glasses Glasses, also known as eyeglasses or spectacles, are vision eyewear, with lenses (clear or tinted) mounted in a frame that holds them in front of a person's eyes, typically utilizing a bridge over the nose and hinged arms (known as temples or ...

and

gels A gel is a semi-solid that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state, although the liquid phase may still di ...

. Bismuth endows materials with a variety of interesting optical properties that can be tuned by changing the supporting material. Commonly-reported structures include the

trigonal bipyramidal In chemistry, a trigonal bipyramid formation is a molecular geometry with one atom at the center and 5 more atoms at the corners of a triangular bipyramid. This is one geometry for which the bond angles surrounding the central atom are not identi ...

cluster, the

octahedral In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at ea ...

cluster, the square antiprismatic cluster, and the

tricapped trigonal prismatic In chemistry, the tricapped trigonal prismatic molecular geometry describes the shape of compounds where nine atoms, groups of atoms, or ligands are arranged around a central atom, defining the vertices of a triaugmented triangular prism (a trigo ...

cluster.

Known materials

Crystalline

* Bi₅(AlCl₄)₃ * Bi₈(AlCl₄)₂ * Bi₅(GaCl₄)₃ * Bi₈(GaCl₄)₂

Metal complexes

* uBi₈AlCl₄]₃ * u(Bi₈)₂sup>6+ * u₂Bi₁₄Br₄AlCl₄]₄

Structure and bonding

Bismuth polycations form despite the fact that they possess fewer total valence electrons than would seem necessary for the number of Sigma bond, sigma bonds. The shapes of these clusters are generally dictated by

Wade's rules In chemistry the polyhedral skeletal electron pair theory (PSEPT) provides electron counting rules useful for predicting the structures of cluster compound, clusters such as borane and carborane clusters. The electron counting rules were originall ...

, which are based on the treatment of the electronic structure as delocalized

molecular orbitals In chemistry, a molecular orbital is a mathematical function describing the location and wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding ...

. The bonding can also be described with three-center two-electron bonds in some cases, such as the cluster. Bismuth clusters have been observed to act as

ligands In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electro ...

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. Bi8_2plus_0p6

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. It is the tendency of electron-deficient bismuth to form sigma-delocalized clusters with small

HOMO/LUMO In chemistry, HOMO and LUMO are types of molecular orbitals. The acronyms stand for ''highest occupied molecular orbital'' and ''lowest unoccupied molecular orbital'', respectively. HOMO and LUMO are sometimes collectively called the ''frontie ...

gaps that gives rise to the near-infrared emissions. This property makes these species potentially valuable to the field of near-infrared optical tomography, which exploits the

near-infrared window in biological tissue The near-infrared (NIR) window (also known as optical window or therapeutic window) defines the range of wavelengths from 650 to 1350 nanometre (nm) where light has its maximum depth of penetration in tissue. Within the NIR window, scattering is t ...

References

{{reflist Bismuth compounds Cations Cluster chemistry