There are three sets of indium halides, the trihalides, the monohalides, and several intermediate halides.
In the monohalides the oxidation state

Oxidation state

In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...

 of indium is +1 and their proper names are indium(I) fluoride, indium(I) chloride, indium(I) bromide and indium(I) iodide.

The intermediate halides contain indium with oxidation state

Oxidation state

In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...

s, +1, +2 and +3.

Indium trihalides

In all of the trihalides the oxidation state

Oxidation state

In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...

 of indium is +3, and their proper names are indium(III) fluoride, indium(III) chloride, indium(III) bromide, and indium(III) iodide. The trihalides are 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,...

ic. Indium trichloride is a starting point in the production of trimethylindium

Trimethylindium

Trimethylindium , In3, is the preferred metalorganic source of Indium for metalorganic vapour phase epitaxy of indium-containing compound semiconductors, such as InP, InAs, InN, InSb, GaInAs, InGaN, AlGaInP, AlInP, AlInGaNP etc. TMI is a white, crystalline and sublimable solid, with melting point...

 which is used in the semiconductor industry.

Indium(III) fluoride

InF₃ is a white crystalline solid with mp 1170 °C. Its structure contains 6 coordinate indium.

Indium(III) chloride

Indium(III) chloride

Indium chloride is the chemical compound with the formula InCl3. This colorless salt finds some use in organic synthesis as a Lewis acid. It is also the most available soluble derivative of indium.-Synthesis and structure:...

and Indium(III) bromide
InCl₃ is a white crystalline solid mp 586 °C. It has the same structure as AlCl₃.

ndium(III) bromide

InBr₃ a pale yellow crystalline solid, m.p. 435 °C. It has the same structure as AlCl₃. InBr₃ finding some use in organic synthesis

Organic synthesis

Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...

 as a water tolerant Lewis acid.

Indium(III) iodide

InI₃ is a coloured crystalline solid, usually described as orange. Distinct yellow and a red forms are known. The red form undergoes a transition to the yellow at 57 °C. The structure of the red form has not been determined by X-Ray crystallography

X-ray crystallography

X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...

, however spectroscopic evidence indicates that indium may be six coordinate. The yellow form consists of In₂I₆ with 4 coordinate indium centres. It is used as an "iodide getter" in the Cativa Process

Cativa process

The Cativa process is a method for the production of acetic acid by the carbonylation of methanol. The technology, which is similar to the Monsanto process, was developed by BP Chemicals and is under license by BP Plc...

.

Intermediate halides

A surprising number of intermediate chlorides and bromides are known, but only one iodide, and no difluoride. Rather than the apparent oxidation state of +2, these compounds contain indium in the +1 and +3 oxidation states. Thus the diiodide is described as In^IIn^IIIX₄. It was some time later that the existence of compounds containing the anion In₂Br₆²⁻ were confirmed which contains an indium-indium bond. Early work on the chlorides and bromides involved investigations of the binary phase diagrams of the trihalides and the related monohalide. Many of the compounds were initially misidentified as many of them are incongruent and decompose before melting. The majority of the previously reported chlorides and bromides have now either had their existence and structures confirmed by X Ray diffraction studies or have been consigned to history. Perhaps the most unexpected case of mistaken identity was the surprising result that a careful reinvestigation of the InCl/InCl₃ binary phase diagram did not find InCl₂.

The reason for this abundance of compounds is that indium forms 4 and 6 coordinate anions containing indium(III) e.g. InBr₄⁻, InCl₆³⁻ as well as the anion In₂Br₆²⁻ that surprisingly contains an indium-indium bond.

In₇Cl₉ and In₇Br₉

In₇Cl₉ is yellow solid stable up to 250^oC that is formulated In^I₆ (In^IIICl₆)Cl₃

In₇Br₉ has a similar structure to In₇Cl₉ and can be formulated as In^I₆ (In^IIIBr₆)Br₃

In₅Br₇

In₅Br₇ is a pale yellow solid. It is formulated In^I₃ (In^II₂Br₆)Br. The In^II₂Br₆ anion has an eclipsed ethane like structure with a metal-metal bond length of 270 pm.

In₂Cl₃ and In₂Br₃

In₂Cl₃ is colourless and is formulated In^I₃ In^IIICl₆ In contrast In₂Br₃ contains the In₂Br₆ anion as present in In₅Br₇, and is formulated In^I(In^II₂Br₆) with a structure similar to Ga₂Br₃.

In₄Br₇

In₄Br₇ is near colourless with a pale greenish yellow tint. It is light sensitive (like TlCl and TlBr) decaying to InBr₂ and In metal. It is a mixed salt containing the InBr₄⁻ and InBr₆³⁻ anions balanced by In⁺ cations. It is formulated In^I₅ (In^IIIBr₄)₂ (In^IIIBr₆) The reasons for the distorted lattice have been ascribed to an antibonding combination between doubly filled, non-directional indium 5s orbitals and neighboring bromine 4p hybrid orbitals.

In₅Cl₉

In₅Cl₉ is formulated as In^I₃In^III₂Cl₉. The In₂Cl₉²⁻ anion has two 6 coordinate indium atoms with 3 bridging chlorine atoms, face sharing bioctahedra, with a similar structure to Cr₂Cl₉²⁻ and Tl₂Cl₉²⁻.

InBr₂ and InI₂

InBr₂ is a greenish white crystalline solid, which is formulated In^IIn^III Br₄. It has the same structure as GaCl₂. InBr₂ is soluble in aromatic solvents and some compounds containing η⁶-arene In(I) complexes have been identified. (See hapticity

Hapticity

The term hapticity is used to describe how a group of contiguous atoms of a ligand are coordinated to a central atom. Hapticity of a ligand is indicated by the Greek character 'eta', η. A superscripted number following the η denotes the number of contiguous atoms of the ligand that are bound to...

 for an explanation of the bonding in such arene-metal ion complexes). With some ligands InBr₂ forms neutral complexes containing an indium-indium bond.
InI₂ is a yellow solid that is formulated In^IIn^IIII₄.

Monohalides

The solid monohalides InCl, InBr and InI are all unstable with respect to water, decomposing to the metal and indium(III) species. They fall between gallium(I) compounds, which are more reactive and thallium(I) that are stable with respect to water. InI is the most stable. Up until relatively recently the monohalides have been scientific curiosities, however with the discovery that they can be used to prepare indium cluster and chain compounds they are now attracting much more interest.

InCl

The room temperature form of InCl is yellow, with a cubic distorted NaCl structure. The red high temperature (>390^oC) has the β-TlI structure.

InBr

InBr is a red crystalline solid, mp 285^oC. It has the same structure as β-TlI, with an orthorhombic distorted rock salt structure. It can be prepared from indium metal and InBr₃.

InI

InI is a deep red purple crystalline solid. It has the same structure as β-TlI. It can be made from the elements at high temperature. Alternatively it can be prepared from InI₃ and indium metal in refluxing xylenes. It is the most stable of the solid monohalides and is soluble in some organic solvents. Solutions of InI in a pyridine/m-xylene mixture are stable below 243 K.

Anionic halide complexes of In(III)

The trihalides are Lewis Acids

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,...

 and form addition compounds with ligands. For InF₃ there are few examples known however for the other halides addition compounds with tetrahedral, trigonal bipyramidal and octahedral coordination geometries are known.
With halide ions there are examples of all of these geometries along with some anions with octahedrally coordinated indium and with bridging halogen atoms, In₂X₉³⁻ with three bridging halogen atoms and In₂X₇⁻ with just one.
Additionally there are examples of indium with square planar geometry in the InX₅^2- ion. The square planar geometry of InCl₅²⁻ was the first found for a main group element.

InX₄⁻ and InX₆³⁻

Salts of InCl₄⁻, InBr₄⁻ and InI₄⁻ are known. The salt LiInF₄ has been prepared however it does not contain tetrahedral anions but has an unusual layer structure with octahedrally coordinated Indium atoms. Salts of InF₆^3-, InCl₆³⁻ and InBr₆³⁻ have all been made.

InCl₅²⁻ and InBr₅²⁻

The InCl₅²⁻ ion has been found to be square pyramidal in the salt (NEt4)₂ InCl₅, with the same structure as (NEt₄)₂ TlCl₅, but is trigonal bipyramidal in tetraphenylphosphonium pentachloroindate acetonitrile solvate

The InBr₅²⁻ ion has similarly been found square pyramidal, albeit distorted, in the Bis(4-chloropyridinium) salt and trigonal bipyramidal in Bi₃₇InBr₄₈

In₂X₇⁻

The In₂X₇⁻ ions contain a single bridging halogen atom. Whether the bridge is bent or linear cannot be determined from the spectra. The chloride and bromide have been detected using electrospray mass spectrometry. The In₂I₇⁻ ion has been prepared in the salt CsIn₂I₇.

In₂X₉³⁻

The caesium salts of In₂Cl₉³⁻ and In₂Br₉³⁻ both contain binuclear anions with octahedrally coordinated Indium atoms.

In^IX₂⁻ and In^IX₃²⁻

In^IX₂^- is produced when the In₂X₆^2- ion disproportionates

Disproportionation

Disproportionation, also known as dismutation is used to describe a specific type of redox reaction in which a species is simultaneously reduced and oxidized so as to form two different products....

. Salts containing the In^IX₃²⁻ ions have been made and their vibrational spectra interpreted as showing that they have C_3v symmetry, trigonal pyramidal

Trigonal pyramid (chemistry)

In chemistry, a trigonal pyramid is a molecular geometry with one atom at the apex and three atoms at the corners of a trigonal base. When all three atoms at the corners are identical, the molecule belongs to point group C3v. One example of a molecule with a trigonal pyramidal geometry is ammonia...

 geometry, with structures similar to the isoelectronic SnX₃⁻ ions.

In₂Cl₆²⁻, In₂Br₆²⁻ and In₂I₆²⁻

Salts of the chloride, bromide and iodide ions ( Bu₄N)₂In₂X₆ have been prepared. In non aqueous solvents this ion disproportionates

Disproportionation

Disproportionation, also known as dismutation is used to describe a specific type of redox reaction in which a species is simultaneously reduced and oxidized so as to form two different products....

 to give In^IX₂⁻ and In^IIIX₄⁻.

Neutral Indium(II) halide adducts

Following the discovery of the In₂Br₆^2- a number of related neutral compounds containing the In^II₂X₄ kernel have been formed from the reaction of indium dihalides with neutral ligands. Some chemists refer to these adducts, when used as the starting point for the synthesis of cluster compounds as ‘In₂X₄’ e.g. the TMEDA adduct.

General references

• WebElements