Indium halides

There are three sets of indium halides, the trihalides, the monohalides and a surprising number of intermediate halides. In all of the trihalides the oxidation state of indium is +3 and their proper names are indium(III) fluoride, indium(III) chloride, indium(III) bromide and indium(III) iodide.
In the monohalides the oxidation state 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 states, +1, +2 and +3.

Indium trihalides

All four trihalides are known. The trihalides are all Lewis acids. Indium trichloride is a starting point in the production of trimethylindium which is used in the semiconductor industry.

;Indium(III) fluoride
:InF₃ is a white crystalline solid with mp 1170^oC. Its structure contains 6 coordinate indium.;Indium(III) chloride and Indium(III) bromide
:InCl₃ is a white crystalline solid mp 586^oC and InBr₃ a white-pale yellow crystalline solid mp 435^o.Both have the same structure as AlCl₃. InBr₃ is finding some use in organic synthesis as a water tolerant Lewis acid. [Zhan-Hui Zhang Thieme Connect Synlett 2005 711 ] . ;Indium(III) iodide
:InI₃ is a coloured crystalline solid, usually described as orange. There are in fact two forms a yellow form and a red form. The red form undergoes a transition to the yellow at 57^o. Surprisingly the crystal structure of the red form has not been determined by X-Ray crystallographic methods, however spectroscopic evidence indicates that indium may be six coordinate with a lattice structure. [Taylor M. J., Kloo L. A. Journal of Raman Spectroscopy 31, 6, (2000), 465] In the yellow form InI₃ is present as dimers, with 4 coordinate indium.

Intermediate halides

There are a surprising number of intermediate chlorides and bromides, only one iodide, InI₂, and no fluorides. The first intermediate halides found were the dihalides where indium has an apparent oxidation state of +2. These were actually found to contain indium in the +1 and +3 oxidation states and to be formulated 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 [Meyer G., Blachnik R.Z. Anorg. Allg. Chem. 1983, 503, 126 ] . 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 [ H.P. Beck D. Wilhelm Angew. Chem., Int. Ed. Engl., 30 (7) (1991) 824-25] :In₇Br₉ has a similar structure to In₇Cl₉ and can be formulated as In^I₆ (In^IIIBr₆)Br₃ [ Dronskowski R Z Kristallogr. 210 (1995) 920]

;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. [ M Ruck , H Bärnighausen Z. Anorg. allg. Chem. 625, Issue 4, (1999), 577] ;In₂Cl₃ and In₂Br₃
:In₂Cl₃ is colourless and is formulated In^I₃ In^IIICl₆ [ Meyer. G. Z. Anorg. Allgem. Chem. 479 (1981) 7 39]
:In contrast In₂Br₃ contains the In₂Br₆ anion as present in In₅Br₇, and is formulated In^I(In^II₂Br₆) with a structure similar to Ga₂Br₃. [ Staffel. T., Meyer. G.,Z Anorg Allgem Chem 552 9 113]
;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. [ R.Dronskowski, Angew. Chem. Int. Ed. Engl. 34 (1995) 1126.] ;In₅Cl₉
:In₅Cl₉ is formulated 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⁻. [ Meyer. G. Z. Anorg. Allgem. Chem. 1978 445 140,]

;InBr₂ and InI₂
:InBr₂ is a greenish white crystalline solid, which is formulated In^IIn^III Br₄. It is 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 for an explanation of the bonding in such arene-metal ion complexes). With some ligands InBr₂ forms neutral complexes containing an indium-indium bond. [Sinclair I., Worrall I.J Can. J. Chem./Rev. can. chim. 60(6): 695-698 (1982)]
: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.
;InF
:InF only known as an unstable gaseous compound.

;InCl
:The room temperature form of InCl is yellow, with a cubic distorted NaCl structure [J.M. Van den Berg Acta Crystallogr 20 (1966) 905] . The red high temperature (>390^oC) has the β-TlI structure [ C. P. J. M. van der Vorst, G. C. Verschoor, W. J. A. Maaskant, Acta Crystallogr. 1978, B34, 3333.] .

;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. [ Organic Syntheses, Coll. Vol. 10, p.170 (2004); Vol. 79, p.59 (2002)] 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. [ Jennifer A. J. Pardo J.A.J., Cowley A.R. , Downs A.J. , Greene T.M. Acta Cryst. (2005). C61, 200]

Anionic Halide complexes of In(III)

The trihalides are Lewis Acids 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 [ Gravereau P, Chaminade J.P, Gaewdang., T., Grannec J., Pouchard M., Hagenmuller P. Acta Cryst. (1992). C48, 769] 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⁻ [ Spiro Inorg Chem 4 1290 (1965) ] 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 [ Bubenheim W., Frenzen G., Muller U. Acta cryst. C, 51, 6, (1995), 1120. ] :The InBr₅²⁻ ion has similarly been found square pyramidal, albeit distorted, in the Bis(4-chloropyridinium) salt [ Ishihara H., Dou S., Gesing T.M., Paulus H., Fuess H., Weiss A. Journal of Molecular Structure 471, 1 (1998)175 ] and trigonal bipyramidal [ Dubenskyy V., Ruck M., Z Anorg. Allgem. Chemie , 629, (2003), 3, 375] 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. [ Taylor M. J. Kloo L. A. Journal of Raman Spectroscopy 31, 6, (2000), 465] ;In₂X₉³⁻ :The caesium salts of In₂Cl₉³⁻ and In₂Br₉³⁻ both contain binuclear anions with octahedrally coordinated Indium atoms. [. Meyer Z. Anorg. allgem. Chem. 1978, 445, 140]

Anionic halide complexes of In(I) and In(II)

;In^IX₂⁻ and In^IX₃²⁻:the In^IX₂^- is produced when the In₂X₆^2- ion disproportionates. Salts containing the In^IX₃²⁻ ions have been made and their vibrational spectra interpreted as showing that they have C_3v symmetry,trigonal pyramidal 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 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 [Sinclair I., Worrall I.J. Can. J. Chem./Rev. can. chim. 60(6): 695-698 (1982)] .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. [ Xiao-Wang Li , Robinson G, Pennington W.T Main Group Chemistry 1, (1996) 301]

General References

* [http://www.webelements.com/ WebElements] Greenwood&Earnshaw
Cotton&Wilkinson6th

Footnotes