The Double SaltsMe+Br·Me2+Br2·6H2O (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+X2·6H2O) in aqueous-halide solutions is established. Two new double salts, KBr·NiBr2·6H2O (PDF 45-196) and RbBr·CoBr2·6H2O (PDF 45-198), are found in the KBr–NiBr2–H2O and RbBr–CoBr2–H2O systems, respectively. The structure of NH4Br·NiBr2·6H2O (1) (PDF 45-193), to which the salts are isopointal, was determined by single crystal X-ray diffraction: monoclinic crystal system, space groupC2/c,a=9.574(2) A,b=9.756(2) A,c=13.668(2) A,β=90.10(2)°,V=1276.6(5) A3,Z=4,Dx=2.208, F.W.=424.56,R=0.050,WR=0.054 for 743 reflections withI>2σ(I). The compounds have a distorted perovskite type structure by analogy with the monoclinic magnesium carnallites. The crystallization of the double saltsMe+X·Me2+X2·6H2O from aqueous-halide solutions is connected with the cations' ability to form the complexes [Me2+(H2O)6] and [Me+X6]. 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 possibleMe+X·Me2+X2·6H2O (Me2+=Ni, Co) double salts is calculated.