Polyiodide

The polyiodides are a class of polyhalogen anions composed entirely of iodine atoms. The most common and simplest member is the triiodide ion, I⁻
₃. Other known, larger polyiodides include [I₄]²⁻, [I₅]⁻, [I₇]⁻, [I₈]²⁻, [I₉]⁻, [I₁₀]²⁻, [I₁₀]⁴⁻, [I₁₁]³⁻, [I₁₂]²⁻, [I₁₃]³⁻, [I₁₆]²⁻, [I₂₂]⁴⁻, [I₂₆]³⁻, [I₂₆]⁴⁻, [I₂₈]⁴⁻ and [I₂₉]³⁻.^[1]

Preparation

The polyiodides can be made by addition of stoichiometric amounts of I₂ to solutions containing I⁻ and I⁻
₃, with the presence of large counter-cations 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.^[2]

Structure

The 14-membered ring array of iodine atoms in [([16]aneS₄)PdIPd([16]aneS₄)][I₁₁

]

The primitive cubic lattice of iodide ions bridge by I₂ molecules, present in [Cp*₂Fe]₄[I₂₆

]

Polyiodides are characterized by their highly complex and variable structures, and can be considered as associations of I₂, I⁻, and I⁻
₃ units. Discrete polyiodides are usually 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 associated cations quite strongly. The table below lists the polyiodide salts which have been structurally characterized, along with their counter-cation.^[3]

Structure of higher polyiodides
Anion	Counter-cation	Structural description
[I₃]⁻	Cs⁺	linear
[I₄]²⁻	[Cu(NH₃)₄]²⁺	symmetric linear array of iodine atoms^[4]
[I₅]⁻	[EtMe₃N]⁺	V-shaped with polymeric layers
[I₅]⁻	[EtMePh₂N]⁺	V-shaped with isolated [I₅]⁻ ions
[I₇]⁻	[Ag(18aneS₆)]⁺	an anionic network derived from a primitive rhombohedral lattice of iodide ions bridged by I₂ molecules
[I₈]²⁻	[Ni(phen)₃]²⁺	regular anionic shapes, can be described as [I⁻ ₃·I₂·I⁻ ₈] or [I⁻ ₃·I⁻ ₅]
[I₉]⁻	[Me₂ⁱPr PhN]⁺	14-membered ring tied by two I₂ bridges to give 10-membered rings
[I₉]⁻	[Me₄N]⁺	non-octahedral, but a twisted "h"-like arrangement of I⁻ ₃ and I₂ units
[I₁₀]²⁻	[Cd(12-crown-4)₂]²⁺	twisted ring configuration with two I⁻ ₃ units linked by two I₂ molecules
[I₁₁]³⁻	[(16aneS₄)PdIPd(16aneS₄)]³⁺	14-membered ring (9.66 × 12.64 Å) around the complex cation, with the rings interlink further to give an infinite 2D sheet
[I₁₂]²⁻	[Ag₂(15aneS₅)₂]²⁺	extended 3D spiral superstructure supported by Ag–I bonds and weak I···S interactions
[I₁₂]²⁻	[Cu(Dafone)₃]²⁺	planar configuration
[I₁₃]³⁻	[Me₂Ph₂N]⁺	consists of zigzag chains of I⁻ and I₂
[I₁₆]²⁻	[Me₂Ph₂N]⁺	centrosymmetric arrangement of [I₇⁻·I₂·I₇⁻]
[I₁₆]²⁻	[ⁱPrMe₂PhN]⁺	the anion forms 14-membered rings catenated by I₂ molecules, which further link into layers with 10- and 14-membered rings
[I₂₂]⁴⁻	[MePh₃P]⁺	two "L"-shaped [I₅]⁻ units linked by an I₂ molecule and completed by two end-on [I₅]⁻ groups
[I₂₆]³⁻	[Me₃S]⁺	consists of [I₅]⁻ and [I₇]⁻ ions with intercalated I₂ molecules
[I₂₆]⁴⁻	Cp*₂Fe⁺	an anionic network derived from a primitive cubic lattice built from I⁻ ions, with I₂ bridges on all edges and systematically removing ¹⁄₁₂ of the I₂ molecules
[I₂₉]³⁻	Cp₂Fe⁺	an anionic 3D network with a cage-like structure of [{(I⁻ ₅)_¹⁄₂·I₂}·{(I²⁻ ₁₂)_¹⁄₂·I₂}·I₂], with [Cp₂Fe]⁺ ions interacting with the anion in the cavities
[I_∞]^δ−	Pyrroloperylene^+•	Infinite polyiodide homopolymer.^[5]

Structures of some polyiodide ions.

References

↑ Housecroft, Catherine E.; Sharpe, Alan G. (2008). "Chapter 17: The group 17 elements". Inorganic Chemistry (3rd ed.). Pearson. p. 547. ISBN 978-0-13-175553-6.
↑ Brauer, G., ed. (1963). "Potassium triiodide". Handbook of Preparative Inorganic Chemistry. 1 (2nd ed.). New York: Academic Press. p. 294.
↑ King, R. Bruce (2005). "Chlorine, Bromine, Iodine, & Astatine: Inorganic Chemistry". Encyclopedia of Inorganic Chemistry (2nd ed.). Wiley. p. 747. ISBN 9780470862100.
↑ Svensson, Per H.; Kloo, Lars (2003). "Synthesis, Structure, and Bonding in Polyiodide and Metal Iodide–Iodine Systems". Chem. Rev. 103 (5): 1649–1684. doi:10.1021/cr0204101.
↑ Madhu, Sheri; Evans, Hayden A.; Doan-Nguyen, Vicky V. T.; Labram, John G.; Wu, Guang; Chabinyc, Michael L.; Seshadri, Ram; Wudl, Fred (4 July 2016). "Infinite Polyiodide Chains in the Pyrroloperylene-Iodine Complex: Insights into the Starch-Iodine and Perylene-Iodine Complexes". Angewandte Chemie International Edition. 55 (28): 8032–8035. doi:10.1002/anie.201601585.

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[InorgChem-1] Housecroft, Catherine E.; Sharpe, Alan G. (2008). "Chapter 17: The group 17 elements". Inorganic Chemistry (3rd ed.). Pearson. p. 547. ISBN 978-0-13-175553-6.

[2] Brauer, G., ed. (1963). "Potassium triiodide". Handbook of Preparative Inorganic Chemistry. 1 (2nd ed.). New York: Academic Press. p. 294.

[Encyclo-3] King, R. Bruce (2005). "Chlorine, Bromine, Iodine, & Astatine: Inorganic Chemistry". Encyclopedia of Inorganic Chemistry (2nd ed.). Wiley. p. 747. ISBN 9780470862100.

[4] Svensson, Per H.; Kloo, Lars (2003). "Synthesis, Structure, and Bonding in Polyiodide and Metal Iodide–Iodine Systems". Chem. Rev. 103 (5): 1649–1684. doi:10.1021/cr0204101.

[5] Madhu, Sheri; Evans, Hayden A.; Doan-Nguyen, Vicky V. T.; Labram, John G.; Wu, Guang; Chabinyc, Michael L.; Seshadri, Ram; Wudl, Fred (4 July 2016). "Infinite Polyiodide Chains in the Pyrroloperylene-Iodine Complex: Insights into the Starch-Iodine and Perylene-Iodine Complexes". Angewandte Chemie International Edition. 55 (28): 8032–8035. doi:10.1002/anie.201601585.