ChemInform Abstract In order to improve the synthesis of perchlorato complexes of Cd(II) and Hg(II), a systematic study of the reactions of Cl2O6 with Cd and Hg salts is performed. As a result, the syntheses of the six title compounds are described (conditions and yields are given in a table). Only (VI) exhibits ionic character and contains (HgCl+)n chains. The structure of (I), space group R3, is related to that of the layered complexes M(ClO4)2 (M: Co, Ni, Zn). The structure of (II) resembles that of HgCl2. In the complexes (III), (IV), and (V), the anions have polymeric arrangements; their structures are analogous to those fo ACuCl3 (A: alkali metal), NO2Co(ClO4)3, and (NO2)2Cu(ClO4)4, respectively.