Synthesis and Structure of Cs 2 [As(CH 3 )Se 3 ] · 2H 2 O and Cs 2 As 4 Se 6 containing the Catena‐hexaselenidotetraarsenate(II, III) Anion $^{1}_{\infty}\rm [As_{4}Se_{6}^{\phantom{\infty}2-}]$

Heating CsAsSe 3 · 0.5H 2 O in methanol to 190 °C leads to formation of black Cs 2 As 4 Se 6 ( 1 ) together with red Cs 4 As 4 Se 8 and a colourless solution from which the substituted caesium selenidoarsenates(V) Cs 2 [As(CH 3 )Se 3 ] · 2H 2 O ( 2 ) and Cs[As(CH 3 ) 2 Se 2 ] can be obtained as crystalline fractions. Cs 2 As 4 Se 6 contains catena‐hexaselenidotetraarsenate(II,III) anions in which cyclic As 3 Se 5 units are linked through corner‐sharing AsSe 3 pyramids into infinite chains $^{1}_{\infty}\rm [As_{4}Se_{6}^{\phantom{\infty}2-}]$ . The C s symmetric As 3 Se 2 five‐membered rings of the As 3 Se 5 units display an envelope conformation with an As‐As bond length 2.528(3) Å and As‐Se distances of 2.449(2) and 2.447(2) Å. C s symmetry is also observed for the discrete tetrahedral [As(CH 3 )Se 3 ] 2− anions of ( 2 ).