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The polyiodides are a class of
polyhalogen anions
Polyhalogen ions are a group of polyatomic cations and anions containing halogens only. The ions can be classified into two classes, isopolyhalogen ions which contain one type of halogen only, and heteropolyhalogen ions with more than one type of h ...
composed entirely of iodine
Iodine is a chemical element with the Symbol (chemistry), symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , ...
atoms. The most common and simplest member is the triiodide
In chemistry, triiodide usually refers to the triiodide ion, . This anion, one of the polyhalogen ions, is composed of three iodine atoms. It is formed by combining aqueous solutions of iodide salts and iodine. Some salts of the anion have been ...
ion, . Other known larger polyiodides include 4sup>2−, 5sup>−, 6sup>2−, 7sup>−, 8sup>2−, 9sup>−, 10sup>2−, 10sup>4−, 11sup>3−, 12sup>2−, 13sup>3−, 14sup>4-, 16sup>2−, 22sup>4−, 26sup>3−, 26sup>4−, 28sup>4− and 29sup>3−. All these can be considered as formed from the interaction of the I–, I2, and building blocks.
Preparation
The polyiodides can be made by addition of stoichiometric amounts of I2 to solutions containing I− and , with the presence of large countercations to stabilize them. For example, KI3·H2O can be crystallized from a saturated solution of KI when a stoichiometric amount of I2 is added and cooled.Structure
] ] Polyiodides are characterized by their highly complex and variable structures, and can be considered as associations of I2, I−, and units. Discrete polyiodides are usually linear molecular geometry, linear, reflecting the origin of the ion. The more complex two- or three-dimensional network structures of chains and cages are formed as the ions interact with each other, with their shapes depending on their associatedcation
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
s quite strongly, a phenomenon named ''dimensional caging''. The table below lists the polyiodide salts which have been structurally characterized, along with their counter-cation.
Reactivity
Polyiodide compounds are generally sensitive to light due to theirphotochemistry
Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet (wavelength from 100 to 400 nm), visible light (400– ...
. The triiodide ion, , is a characteristic and well-studied triatomic
Triatomic molecules are molecules composed of three atoms, of either the same or different chemical elements. Examples include H2O, CO2 (pictured), HCN and O3 (ozone)
Molecular vibrations
The vibrational modes of a triatomic molecule can be d ...
system undergoing unimolecular photodissociation
Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...
. Polyiodide has been used to improve the scalability in the synthesis of halide perovskite
Perovskite (pronunciation: ) is a calcium titanium oxide mineral composed of calcium titanate (chemical formula ). Its name is also applied to the class of compounds which have the same type of crystal structure
In crystallography, crystal ...
photovoltaic materials.
Conductivity
Solid state compounds containing linear-chain polyiodideion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
s exhibit enhanced conductivity than their simple iodide
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. In everyday life, iodide is most commonly encountered as a component of iodized salt, which many governments mandate. Worldwide, iodine def ...
counterparts. The conductivity can be drastically modified by external pressure, which changes the interatomic distances between iodine moieties and the charge distribution.{{Cite journal , last=Poręba , first=Tomasz , last2=Ernst , first2=Michelle , last3=Zimmer , first3=Dominik , last4=Macchi , first4=Piero , last5=Casati , first5=Nicola , date=2019-05-13 , title=Pressure‐Induced Polymerization and Electrical Conductivity of a Polyiodide , url=https://onlinelibrary.wiley.com/doi/10.1002/anie.201901178 , journal=Angewandte Chemie International Edition , language=en , volume=58 , issue=20 , pages=6625–6629 , doi=10.1002/anie.201901178 , issn=1433-7851
See also
*Triiodide
In chemistry, triiodide usually refers to the triiodide ion, . This anion, one of the polyhalogen ions, is composed of three iodine atoms. It is formed by combining aqueous solutions of iodide salts and iodine. Some salts of the anion have been ...
* Polyhalogen ions
Polyhalogen ions are a group of polyatomic cations and anions containing halogens only. The ions can be classified into two classes, isopolyhalogen ions which contain one type of halogen only, and heteropolyhalogen ions with more than one type of ...
* Iodine–starch test
* Dye-sensitized solar cell
* Halogen bond A halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. Like a hydrogen ...
* Catenation
In chemistry, catenation is the bonding of atoms of the same element into a series, called a ''chain''. A chain or a ring shape may be ''open'' if its ends are not bonded to each other (an open-chain compound), or ''closed'' if they are bond ...
* Inorganic polymer
An inorganic polymer is a polymer with a skeletal structure that does not include carbon atoms in the backbone. Polymers containing inorganic and organic components are sometimes called hybrid polymers, and most so-called inorganic polymers are ...
References