In chemistry,
triiodide (sometimes written tri-iodide) can have several meanings. Triiodide primarily refers to the triiodide ion, I
3−, a
polyatomicA 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
iodineIodine , is a chemical element that has the symbol I and atomic number 53. Naturally-occurring iodine is a single isotope with 74 neutrons....
atoms. For some chemical compounds, triiodide indicates a salt of the named cation with the triiodide anion. Examples include sodium triiodide,
thallium triiodideThallium 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−.An appreciation as to...
, and
ammonium triiodideAmmonium 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 ....
, [NH
4][I
3].
In chemistry,
triiodide (sometimes written tri-iodide) can have several meanings. Triiodide primarily refers to the triiodide ion, I
3−, a
polyatomicA 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
iodineIodine , is a chemical element that has the symbol I and atomic number 53. Naturally-occurring iodine is a single isotope with 74 neutrons....
atoms. For some chemical compounds, triiodide indicates a salt of the named cation with the triiodide anion. Examples include sodium triiodide,
thallium triiodideThallium 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−.An appreciation as to...
, and
ammonium triiodideAmmonium 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 ....
, [NH
4][I
3]. Each of these compounds has a (mono)
iodideAn iodide ion is an iodine atom with a −1 charge. Compounds with iodine in formal oxidation state −1 are called iodides. This can include ionic compounds such as caesium iodide or covalent compounds such as phosphorus triiodide. This is the same naming scheme as is seen with chlorides...
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 triiodideNitrogen triiodide, also called nitrogen iodide or, more correctly, triiodine nitride to distinguish it from the triiodide ion, is the chemical compound with the formula NI3. It is an extremely sensitive contact explosive: small quantities explode with a gunpowder-like snap when touched...
(NI
3),
phosphorus triiodidePhosphorus 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...
,
antimony triiodideAntimony 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. Like many iodides of the heavier main group elements, its...
, and
gallium triiodideGallium 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...
(Ga
2I
6). Some anions have the theoretical possibility to form either kind of triiodide.
Thallium triiodideThallium 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−.An appreciation as to...
is described as thallium(I) triiodide; thallium(III) iodide is unknown.
Triiodide ion
The triiodide ion is the simplest
polyiodideThe 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
solventA solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution.The most common solvent in everyday life is water. Most other commonly-used solvents are organic chemicals. These are called organic solvents...
s.
Lugol's iodineLugol'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...
contains potassium iodide as well, so that significant amounts of triiodide ion can exist in solution.
Tincture of iodineTincture of iodine is a disinfectant, usually 2-7% potassium iodide or sodium iodide, and elemental iodine, 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 turning...
, 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
ionAn ion is an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge...
:
- I2 + I− I3−
In this reaction, iodide is viewed as a Lewis base, and the
iodineIodine , is a chemical element that has the symbol I and atomic number 53. Naturally-occurring iodine is a single isotope with 74 neutrons....
is a
Lewis acidA Lewis acid, A, is a chemical compound that can accept a pair of electrons from a Lewis base, B, that acts as an electron-pair donor, forming an adduct, AB:
A + :B → A—B...
. The process is analogous to the reaction of
S8Sulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant, multivalent non-metal. Sulfur, in its native form, is a yellow crystalline solid. In nature, it can be found as the pure element and as sulfide and sulfate minerals...
with
sodium sulfideSodium sulfide is the name used to refer to the chemical compound Na
2S but more commonly its hydrate Na
2S
.9H
2O. 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 theoryValence 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 bondThe 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 I
a–I
b–I
c bonds in a few sample compounds are shown below:
| compound |
Ia–Ib (pm) |
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 |
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