Arsenite

In chemistry, an arsenite is a chemical compound containing an arsenicoxyanion where arsenic has oxidation state +3. Note that in fields that commonly deal with groundwater chemistry, arsenite is used generically to identify soluble As anions. IUPAC have recommended that arsenite compounds are to be named as arsenate(III), for example orthoarsenite is called trioxidoarsenate(III). Orthoarsenite contrasts to the corresponding anions of the lighter members of group 15, phosphite which has the structure HPO2−3 and nitrite, NO−2 which is bent.

A number of different arsenite anions are known:

• AsO3−3 orthoarsenite, an ion of arsenous acid, with a pyramidal shape
• (AsO−2)_n metaarsenite, a polymeric chain anion.
• As₂O4−5 pyroarsenite, [O₂As−O−AsO₂]
• As₃O5−7catena-triarsenite, [O₂As−O−As(O)−O−AsO₂]
• As₄O6−9catena-tetraarsenite, [O₂As−O−As(O)−O−As(O)−O−AsO₂]
• As₄O4−8cyclo-tetraarsenite
• (As₆O4−11)_n, a polymeric anion

In all of these the geometry around the As centers are approximately trigonal, the lone pair on the arsenic atom is stereochemically active. Well known examples of arsenites include sodium metaarsenite which contains a polymeric linear anion, (AsO−2)_n, and silver orthoarsenite, Ag₃AsO₃, which contains the trigonal AsO3−3 anion.