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 (enH 2)[ M(HSeO 3) 2 X 2] compounds are the most commonly formed species which constitute a significant contribution to the family of layered hydrogen selenites containing neutral [ M(HSeO 3) 2] ( M = Mg, Mn, Co, Ni, Cu, Zn, Cd) 2 D 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 (pipH 2)[ M(HSeO 3) 2 X 2] compounds are formed only with M = 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 H 2SeO 3, HSeO 3 -, SeO 3 2-, and Se 2O 5 2-, reflecting amazing adaptability to the shape of the templating cations.