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Systematic studies of crystalline compounds formed in aqueous systems containing aliphatic diamines, divalent transition metal halides, and selenious acid resulted in the discovery of a large family of new complex species corresponding to several new structure types. With ethylenediamine (en), layered (enH₂)[<i>M</i>(HSeO₃)₂<i>X</i>₂] compounds are the most commonly formed species which constitute a significant contribution to the family of layered hydrogen selenites containing neutral [<i>M</i>(HSeO₃)₂] (<i>M</i> = Mg, Mn, Co, Ni, Cu, Zn, Cd) 2<i>D</i> building blocks. In contrast to some previous suggestions, piperazine (pip), as well as its homologue N-methylpiperazine, mostly give rise to quite different, sometimes more complex, structures of varied dimensionality while the (pipH₂)[<i>M</i>(HSeO₃)₂<i>X</i>₂] compounds are formed only with <i>M</i> = Cu and Cd. In addition, metal-, halide-, or selenium-free by-product species are observed. The Se^IV can be present in a multitude of forms, including H₂SeO₃, HSeO₃⁻, SeO₃²⁻, and Se₂O₅²⁻, reflecting amazing adaptability to the shape of the templating cations.