The polyiodides are a class of polyatomic

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...

 halide

Halide

A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...

 anions composed entirely of 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....

 atom

Atom

The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s.

The triiodide

Triiodide

In chemistry, triiodide 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, ...

 ion, I₃⁻, is the simplest polyiodide. Larger polyiodides are known, with single or multiple negative charges.

The basic 'building blocks' of polyiodides can be considered as I₂, I⁻, and I₃⁻. The more complex polyiodides can be made by addition of I₂ to solutions containing I⁻ and I₃⁻, with the condition of presence of large cations to stabilise them. With cations much smaller than NMe₄⁺

Quaternary ammonium cation

Quaternary ammonium cations, also known as quats, are positively charged polyatomic ions of the structure NR4+, R being an alkyl group or an aryl group. Unlike the ammonium ion and the primary, secondary, or tertiary ammonium cations, the quaternary ammonium cations are permanently charged,...

, it is either not possible to precipitate

Precipitation (chemistry)

Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...

 the salt (for example with Na⁺ or K⁺ cations), or the crystal structure will show asymmetric iodide anions, as in caesium triiodide, CsI₃.

The shapes of the polyiodides depend on their associated cations quite strongly, however some of the simpler ions have roughly constant shapes:

• I₅⁻ normally adopts a V-shaped structure and can be regarded as two I₂ molecules attached to an I⁻ ion. Bonding in this species suggests p-orbital interactions, and the bond lengths of approximately 2.8 Å (0.28 nm
  Nanometre
  A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...
  
  ) for the terminal bonds and 3.17 Å (0.317 nm
  Nanometre
  A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...
  
  ) for the centre bonds supports the I₂ and I⁻bonding model.
• I₄²⁻ is notionally composed of either two I₂ species, or is almost equally bonded, depending on environment of the ion. Both models are linear.