Manganese(II) Coordination Polymers [{Mn(tren)}(As2Se4)] and [{Mn(tren)}(As4Se7)] with Infinite Selenidoarsenate(III) Chains $^{1}_{infty}rm [AsSe_{2}^{-}]$ and $^{1}_{infty}rm [As_{4}Se_{7}^{2-}]$

Solvothermal reaction of [MnCl₂(tren)] with elemental As and Se at 1:1:2 and 1:6:12 molar ratios in H₂O/tren (10:1) affords the 1D coordination polymers [{Mn(tren)}(As₂Se₄)] ( 1 ) and [{Mn(tren)}(As₄Se₇)] ( 2 ), respectively. 1 contains vierer infinite chains, which coordinate [(tren)Mn]²⁺ fragments through every second terminal Se atom of their corner‐sharing pyramidal AsSe₃ building units. The double chains of compound 2 are related to the chains 1 by a simple rearrangement of the connectivity pattern between the participating AsSe₃ pyramids and contain condensed centrosymmetric As₈Se₈ and As₄Se₄ rings.