Triiodide
Encyclopedia
In chemistry, triiodide (sometimes written tri-iodide) can have several meanings. Triiodide primarily refers to the triiodide ion, I3−, a polyatomic
anion composed of three iodine
atoms. For some chemical compounds, triiodide indicates a salt of the named cation with the triiodide anion. Examples include sodium triiodide, thallium triiodide
, and ammonium triiodide
, [NH4][I3]. Each of these compounds has a (mono)iodide
counterpart. For other chemical compounds, triiodide indicates that each molecule contains three iodine atoms that are not bonded to each other, not forming the triiodide ion. Examples include nitrogen triiodide
(NI3), phosphorus triiodide
, antimony triiodide
, and gallium triiodide
(Ga2I6). Some anions have the theoretical possibility to form either kind of triiodide. Thallium triiodide
is described as thallium(I) triiodide; thallium(III) iodide is unknown.
; several higher polyiodides exist. In solution, it appears yellow in low concentration, and brown at higher concentration. The triiodide ion is responsible for the well-known blue-black color which arises when iodine solutions react with starch. Iodide does not react with starch; nor do solutions of iodine in nonpolar solvent
s. Lugol's iodine
contains potassium iodide as well, so that significant amounts of triiodide ion can exist in solution.
Tincture of iodine
, although nominally a solution of elemental iodine in ethanol, also contains significant amounts of triiodide, due to its content of both iodide and water.
:
In this reaction, iodide is viewed as a Lewis base, and the iodine
is a Lewis acid
. The process is analogous to the reaction of S8
with sodium sulfide
, except that the higher polyiodides have branched structures.
The ion is linear, as predicted by VSEPR theory
. A common explanation for the hypervalent bonding on the central atom involves a three-center four-electron bond
. The bond lengths and angles of triiodide vary, depending on the compound. The dimensions of the tri-iodide Ia– Ib– Ic bonds in a few sample compounds are shown below:
Polyatomic ion
A polyatomic ion, also known as a molecular ion, is a charged species composed of two or more atoms covalently bonded or of a metal complex that can be considered as acting as a single unit in the context of acid and base chemistry or in the formation of salts. The prefix "poly-" means "many," in...
anion composed of three iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
atoms. For some chemical compounds, triiodide indicates a salt of the named cation with the triiodide anion. Examples include sodium triiodide, thallium triiodide
Thallium triiodide
Thallium triiodide is a chemical compound of thallium and iodine with formula TlI3. Unlike the other thallium trihalides, which contain thallium, TlI3 is a thallium compound and contains the triiodide ion, I3−....
, and ammonium triiodide
Ammonium triiodide
Ammonium triiodide is the salt of the ammonium cation with the triiodide anion.Sometimes the name ammonium triiodide is mistakenly used to refer to a different compound, nitrogen triiodide ....
, [NH4][I3]. Each of these compounds has a (mono)iodide
Iodide
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
counterpart. For other chemical compounds, triiodide indicates that each molecule contains three iodine atoms that are not bonded to each other, not forming the triiodide ion. Examples include nitrogen triiodide
Nitrogen triiodide
Nitrogen triiodide is the inorganic compound with the formula NI3. It is an extremely sensitive contact explosive: small quantities explode with a gunpowder-like snap when touched even lightly, releasing a purple cloud of iodine vapor...
(NI3), phosphorus triiodide
Phosphorus triiodide
Phosphorus triiodide is an unstable red solid which reacts violently with water. It is a common misconception that PI3 is too unstable to be stored; it is, in fact, commercially available. It is widely used in organic chemistry for converting alcohols to alkyl iodides. It is also a powerful...
, antimony triiodide
Antimony triiodide
Antimony triiodide is the chemical compound with the formula SbI3. This ruby-red solid is the only characterized "binary" iodide of antimony, i.e. the sole compound isolated with the formula SbxIy. It contains antimony in its +3 oxidation state. Like many iodides of the heavier main group...
, and gallium triiodide
Gallium(III) iodide
Gallium iodide is the chemical compound with the formula Ga2I6. It is the most common iodide of gallium. In the chemical vapor transport method of growing crystals of gallium arsenide uses iodine as the transport agent...
(Ga2I6). Some anions have the theoretical possibility to form either kind of triiodide. Thallium triiodide
Thallium triiodide
Thallium triiodide is a chemical compound of thallium and iodine with formula TlI3. Unlike the other thallium trihalides, which contain thallium, TlI3 is a thallium compound and contains the triiodide ion, I3−....
is described as thallium(I) triiodide; thallium(III) iodide is unknown.
Triiodide ion
The triiodide ion is the simplest polyiodidePolyiodide
The polyiodides are a class of polyatomic halide anions composed entirely of iodine atoms.The triiodide ion, I3−, is the simplest polyiodide. Larger polyiodides are known, with single or multiple negative charges....
; several higher polyiodides exist. In solution, it appears yellow in low concentration, and brown at higher concentration. The triiodide ion is responsible for the well-known blue-black color which arises when iodine solutions react with starch. Iodide does not react with starch; nor do solutions of iodine in nonpolar solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
s. Lugol's iodine
Lugol's iodine
Lugol's iodine, also known as Lugol's solution, first made in 1829, is a solution of elemental iodine and potassium iodide in water, named after the French physician J.G.A. Lugol. Lugol's iodine solution is often used as an antiseptic and disinfectant, for emergency disinfection of drinking water,...
contains potassium iodide as well, so that significant amounts of triiodide ion can exist in solution.
Tincture of iodine
Tincture of iodine
Tincture of iodine is a disinfectant, usually 2–7% elemental iodine, along with potassium iodide or sodium iodide, dissolved in a mixture of ethanol and water. As in the case of Lugol's iodine, the role of iodide and water in the solution is to increase the solubility of the elemental iodine, by...
, although nominally a solution of elemental iodine in ethanol, also contains significant amounts of triiodide, due to its content of both iodide and water.
Formation and structure
The following endergonic equilibrium gives rise to the triiodide ionIon
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
:
- I2 + I− I3−
In this reaction, iodide is viewed as a Lewis base, and the iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
is a Lewis acid
Lewis acid
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...
. The process is analogous to the reaction of S8
Octasulfur
Octasulfur is a cyclosulfane with the molecular formula S8. It is a simple yellow coloured sulfur. It is also the final member of the thiocane heterocylic series, where every carbon is substituted with a sulfur atom....
with sodium sulfide
Sodium sulfide
Sodium sulfide is the name used to refer to the chemical compound Na2S, but more commonly it refers to the hydrate Na2S·9H2O. Both are colorless water-soluble salts that give strongly alkaline solutions...
, except that the higher polyiodides have branched structures.
The ion is linear, as predicted by VSEPR theory
VSEPR theory
Valence shell electron pair repulsion theory is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. It is also named Gillespie–Nyholm theory after its two main developers...
. A common explanation for the hypervalent bonding on the central atom involves a three-center four-electron bond
Three-center four-electron bond
The 3-center 4-electron bond is a model used to explain bonding in hypervalent molecules such as phosphorus pentafluoride, sulfur hexafluoride, the xenon fluorides, and the bifluoride ion. It is also known as the Pimentel-Rundle three-center model after the work published by George C. Pimentel in...
. The bond lengths and angles of triiodide vary, depending on the compound. The dimensions of the tri-iodide Ia
| compound | Ia |
Ib–Ic (pm) | angle (°) |
|---|---|---|---|
| TlI3 | 306.3 | 282.6 | 177.9 |
| RbI3 | 305.1 | 283.3 | 178.11 |
| CsI3 | 303.8 | 284.2 | 178.00 |
| NH4I3 | 311.4 | 279.7 | 178.55 |