Crystal structure and thermal behavior of nedocromil nickel octahydrate

The hydration behavior of a salt depends on the nature of the cation and the anion and on the molecular packing. A transition metal salt (nickel) of nedocromil was prepared and its crystal structure was elucidated in an attempt to study the influence of the nature of the bivalent cation on the structure, water interactions and molecular packing. Crystal data: nedocromil nickel octahydrate (NNi), orthorhombic, Pca2₁, a = 29.5446(1) Å, b = 25.0444(1) Å, c = 13.3767(2) Å, Z = 16. The Ni²⁺, has octahedral coordination, but the coordination environments of the cations and the bonding environments of the water molecules differ. NNi contains four Ni²⁺ ions in the asymmetric unit, two of which are each octahedrally coordinated to five water molecules and to a carboxyl oxygen. The two remaining Ni²⁺ ions are linked in a Ni₂(H₂O)₁₀⁺⁴ species. Thermal analytical data for NNi show that the water molecules in this hydrate are lost in a single step dehydration, which may be attributed to the fairly continuous water layer in the ac plane of the crystal lattice.