Astatine - Compounds

Compounds

Astatine is the least reactive of the halogens, being less reactive than iodine; however, multiple compounds of astatine have been synthesized in microscopic amounts and studied as intensively as possible before their inevitable radioactive disintegration. The reactions involved are normally tested with dilute solutions of astatine mixed with larger amounts of iodine. The iodine acts as a carrier, ensuring that there is sufficient material for laboratory techniques (such as filtration and precipitation) to work.

The formation of an astatine compound with hydrogen – usually referred to as hydrogen astatide – was noted by the pioneers of astatine chemistry. As mentioned earlier, there are grounds for referring to this compound as astatine hydride instead – astatine is easily oxidized, acidification by (dilute) nitric acid gives the At0 or At+ forms, and the addition of silver(I) then precipitates astatine, only partially as silver(I) astatide (AgAt) (or not at all). Iodine, in contrast, is not oxidized, and precipitates readily as silver(I) iodide.

Only a few metal astatides have been reported, including those of sodium, palladium, silver, and lead. Some characteristic properties of silver astatide, and the known and hypothetical alkali and alkaline earth astatides, have been estimated by extrapolation from other silver or alkali or alkaline earth halides.

Astatine is known to react with its lighter homologues iodine, bromine, and chlorine in the vapor state; these reactions produce diatomic interhalogen compounds with formulas AtI, AtBr, and AtCl. The first two compounds may also be produced in water – astatine reacts with iodine/iodide solution to form AtI, whereas AtBr requires (aside from astatine) an iodine/iodine monobromide/bromide solution. The excess of iodides or bromides may lead to AtBr−
2 and AtI−
2 ions, or in a chloride solution, they may produce species like AtCl−
2 or AtBrCl− via equilibrium-balanced reactions with the chlorides. Oxidation of the element with dichromate (in nitric acid solution) showed that adding chloride turned the astatine into a molecule likely to be either AtCl or AtOCl. Similarly, AtOCl−
2 or AtCl−
2 may be produced. In a plasma ion source mass spectrometer, the similar ions +, +, and + have been formed by introducing lighter halogen vapors into a helium-filled cell containing astatine, supporting the existence of stable neutral molecules in the plasma ion state. No astatine fluorides have been discovered as yet. Their absence has been speculatively attributed to the extreme reactivity of such compounds, including the reaction of an initially-formed fluoride with the walls of the glass container to form a non-volatile product. Thus, although the synthesis of an astatine fluoride is thought to be possible, it may require a liquid halogen fluoride solvent, as has already been used for the characterization of radon fluorides.

With oxygen, there is evidence for the existence of the species AtO–, AtO−
2 and AtO+ in aqueous solution, formed by the reaction of astatine with an oxidant such as elemental bromine or (in the last case) by sodium persulfate in a solution of perchloric acid. The well characterized AtO−
3 anion can be obtained by, for example, the oxidation of astatine with potassium hypochlorite in a solution of potassium hydroxide. Further oxidation, such as by xenon difluoride (in a hot alkaline solution) or periodate (in a neutral or alkaline solution), yields the perastatate ion AtO−
4; however, this is only stable in neutral or alkaline solutions. Astatine is also thought to be capable of forming cationic salts with oxyanions such as iodate or dichromate; this is based on the observation that, in acidic solutions, monovalent or intermediate positive states of astatine coprecipitate with the insoluble salts of metal cations such as silver(I) iodate or thallium(I) dichromate.

Astatine may form bonds to the other chalcogens; these include S₇At+ and At(CSN)−
2 with sulfur, a coordination selenourea compound with selenium, and an astatine–tellurium colloid with tellurium. Additionally, astatine is known to bind to nitrogen, lead, and boron under the proper conditions.

Carbon tetraastatide (CAt₄) is known. Astatine can replace a hydrogen atom in benzene to form C₆H₅At; this may be oxidized to C₆H₅AtCl₂ by chlorine. By treating this compound with an alkaline solution of hypochlorite, C₆H₅AtO₂ can be produced.