Arsenite

In chemistry, an arsenite is a chemical compound containing an arsenic oxyanion where arsenic has oxidation state +3. Note that in fields that commonly deal with groundwater chemistry, arsenite is used generically to identify soluble As^III anions. IUPAC have recommended that arsenite compounds are to be named as arsenate(III), for example ortho-arsenite is called trioxidoarsenate(III). Ortho-arsenite 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.^[1]

A number of different arsenite anions are known:

• AsO3−3 ortho-arsenite, an ion of arsenous acid, with a pyramidal shape^[1]
• (AsO−2)_n meta-arsenite, a polymeric chain anion.^[2]
• As₂O4−5 pyro-arsenite, [O₂As−O−AsO₂]⁴⁻
• As₃O5−7 catena-triarsenite, [O₂As−O−As(O)−O−AsO₂]^5−[3]
• As₄O6−9 catena-tetraarsenite, [O₂As−O−As(O)−O−As(O)−O−AsO₂]^6−[3]
• As₄O4−8 cyclo-tetraarsenite^[4]
• (As₆O4−11)_n, a polymeric anion

In all of these the geometry around the As^III centers are approximately trigonal, the lone pair on the arsenic atom is stereochemically active.^[1] Well known examples of arsenites include sodium meta-arsenite which contains a polymeric linear anion, (AsO−2)_n, and silver ortho-arsenite, Ag₃AsO₃, which contains the trigonal AsO3−3 anion.