Double salts in the systems Me+X-Me2+X2-H2O (Me+ = K, NH4, Rb, Cs; Me2+ = Mn, Co, Ni; X- = Cl, Br)

The solubility diagrams of the systems CsBr-Me2+Br2-H2O (Me2+ = Mn, Co, Ni) and NH4Br-MnBr2-H2O have been determined at 25 degrees C. Four previously unreported double salts, 2NH(4)Br . MnBr2 . 2H(2)O (space group P4(2)/mnm, a = 7.886(1) Angstrom, b = 7.886(1) Angstrom, 8.529(2)Angstrom, Z=2, V=530. 4(2)Angstrom(3), D-x=2.797), 2CsBr . MnBr2. 2H(2)O (space group P (1) over bar, a 5.993(1)Angstrom, b = 7.054(2)Angstrom, c = 7.587(2)Angstrom, alpha = 66.02(2)degrees, beta 87.87(2), gamma = 83.95(2)degrees, Z = 1, V = 291.4 (1)Angstrom(3), D-x = 3.854), CsBr . MnBr2. 2H(2)O (space group Pcca, a 9.5 23(2)Angstrom, b = 7.514(1)Angstrom, c = 11.951(3)Angstrom Z = 4, V = 855.2 (3)Angstrom(3), D-x = 3.600), and KCl . NiCl2. 2H(2)O, have been identified . The crystallization of double salts in the systems Me+X-Me2+X2-H2O (Me+ = K, NH4, Rb, Cs; Me2+ = Mn, Co, Ni; X- = Cl, Br) is explained using the Pau ling rules of formation and arrangement of the coordination polyhedra in st able ionic crystal structures. The type of ligands that form polyhedra is d etermined using Pearson's "hard" and "soft" acids-bases concept. Compositio n and structural differences in the crystallized double salts are explained by the differences in stabilities of the Me2+-polyhedra that predominate i n the saturated solutions. (C) 1999 Academic Press.