The double salts Me+Br center dot Me2+Br2 center dot 6H(2)O (Me+=K, NH4, Rb; Me2+=Co, Ni)

The ability of the Co2+ and Ni2+ ions to form double salts of the carnallite formula type (Me+X . Me2+X2 . 6H(2)O) in aqueous-halide solutions is established. Two new double salts, KBr . NiBr2 . 6H(2)O (PDF 45-196) and RbBr . CoBr2 . 6H(2)O (PDF 45-198), are found in the KBr-NiBr2-H2O and RbBr-CoBr2-H2O systems, respectively. The structure of NH4Br . NiBr2 . 6H(2)O (1) (PDF 45-193), to which the salts are isopointal, was determined hy single crystal X-ray diffraction: monoclinic crystal system, space group C2/c, a = 9.574(2) Angstrom, b = 9.756(2) Angstrom, c = 13.668(2) Angstrom, beta = 90.10(2)degrees, V = 1276.6(5) Angstrom(3), Z = 4, Dx = 2.208, F.W. = 424.56, R = 0.050, WR = 0.054 for 743 reflections with I > 2 sigma(I). The compounds have a distorted perovskite type structure by analogy with the monoclinic magnesium carnallites. The crystallization of the double salts Me+X . Me2+X2 . 6H(2)O from aqueous-halide solutions is connected with the cations' ability to form the complexes [Me2+(H2O)(6)] and [Me+X-6]. Geometric and metal-ligand interaction factors that affect the formation of these complexes are discussed. The ''hard-soft'' acid/base factor is used as a measure for metal-ligand interaction. The geometrical tolerance factor t of all possible Me+X . Me2+X2 . 6H(2)O (Me2+ = Ni, Co) double salts is calculated. (C) 1997 Academic Press.