New polyselenidoarsenate salts with transition metal complexes as counterions: solvothermal synthesis, crystal structures, and properties of [M(dien)2]As2Se6 (M = Co, Ni) and [Mn(dap)3]As2Se6

New polyselenidoarsenates [Co(dien)2]As2Se6, [Ni(dien)2]As2Se6 (dien = diethylenetriamine), and [Mn(dap)3]As2Se6 (dap = 1,2-diaminopropane) were synthesized by the reaction of As2O3, Se, and transition metal chloride in dien or dap solvent under solvothermal conditions. The compounds consist of a polyselenidoarsenate anion [As2Se6]2− and an octahedral complex cation [M(dien)2]2+ (M = Co, Ni) or [Mn(dap)3]2+. Two trigonal–pyramidal AsSe3 units are joined through two Se–Se bonds forming the dimeric [As2Se6]2− anion with a six-membered ring As2Se4 in chair conformation. In the crystal structures the anion and cations are connected into extended structures via weak N–H···Se hydrogen bonds. Thermogravimetric analyses show that the dien-containing compounds decompose in one step under nitrogen stream, whereas the Mn compound exhibits a two-step decomposition process.