Double Salts in the SystemsMe+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°C. Four previously unreported double salts, 2NH4Br·MnBr2·2H2O (space group P42/mnm,a=7.886(1) ?,b=7.886(1) ?,c=8.529(2) ?,Z=2,V=530.4(2) ?3,Dx=2.797), 2CsBr·MnBr2·2H2O (space groupP ,a=5.993(1) ?,b= 7.054(2) ?,c=7.587(2) ?,=66.02(2)°,=87.87(2),=83.95(2)°,Z=1,V=291.4(1) ?3,Dx=3.854), CsBr·MnBr2· 2H2O (space groupPcca,a=9.523(2) ?,b=7.514(1) ?,c=11.951(3) ?,Z=4,V=855.2(3) ?3,Dx=3.600), and KCl· NiCl2·2H2O, have been identified. The crystallization of double salts in the systemsMe+X–Me2+X2–H2O (Me+=K, NH4, Rb, Cs;Me2+=Mn, Co, Ni;X-=Cl, Br) is explained using the Pauling rules of formation and arrangement of the coordination polyhedra in stable ionic crystal structures. The type of ligands that form polyhedra is determined using Pearsonʹs "hard" and "soft" acids–bases concept. Composition and structural differences in the crystallized double salts are explained by the differences in stabilities of theMe2+–polyhedra that predominate in the saturated solutions.