Arsenide

In chemistry, an arsenide is a compound of arsenic with a less electronegative element or elements. Many metals form binary compounds containing arsenic, and these are called arsenides. They exist with many stoichiometries, and in this respect arsenides are similar to phosphides, borides, silicides and nitrides.

The group 1 alkali metals and the group 2, alkaline earth metals, form arsenides with isolated arsenic atoms. They form upon heating arsenic powder with excess sodium gives sodium arsenide (Na₃As). The structure of Na₃As is complex with unusually short Na–Na distances of 328–330 pm which are shorter than in sodium metal. This short distance indicates the complex bonding in these simple phases, i.e. they are not simply salts of As³⁻ anion, for example. The compound LiAs, has a metallic lustre and electrical conductivity indicating some metallic bonding. These compounds are mainly of academic interest. For example, "sodium arsenide" is a structural motif adopted by many compounds with the A₃B stoichiometry.

Indicative of their salt-like properties, hydrolysis of alkali metal arsenides gives arsine:

A number of arsenide minerals are known, for example nickeline, NiAs, skutterudite, CoAs₃ (a diamagnetic semiconductor) Assigning formal oxidation numbers (states) is difficult unless the structure is known, for example the mineral sperrylite (PtAs₂), platinum arsenide, has a formal oxidation state for arsenic of –2. as the solid is usually described as Pt⁴⁺,As₂⁴⁻.

The arsenides of the transition metals are mainly of interest because they represent or contaminate sulfidic ores of commercial interest. The extraction of the metals - nickel, iron, cobalt, copper - entails chemical processes such as smelting that poses environmental risks. In the mineral, arsenic is immobile and poses no environmental risk. Released from the mineral, arsenic is a poisonous and mobile.