Correlation between solution and the solid state:: The pH dependent composition in the ternary system [Co(H2O)6]2+ or [Ni(H2O)6]2+/piperazine/phosphate

The protolytic equilibria of piperazine (C4H10N2) and phosphate have been investigated in the presence of cobalt or nickel chloride or nitrate by potentiometric titrations between pH 2 and 8. Potentiometric titrations suggest the presence of [M2+(H2O)(5)(C4H11N2+)](3-) and [M2-(H2O)(5)(C4H10N2)](2+) in solution with stability constants logK of 3.1 and 3.8 for M = Co and 3.1 and 3.6 for M = Ni, respectively. Crystallization experiments were then conducted at selected pH values to isolate desired species from the known solution composition. Crystallization afforded (M(H2O)(6)](2+)(C4H12N22+)(HPO42-)(2) at pH 3.5 and 6.2 (M = Co, Ni), and Co-3(PO4)(2).8H(2)O at pH 10.5. No crystals with the dihydrogenphosphate anion or a metal-bound piperazine ligand could be isolated under the reaction conditions. The solid-state assembly in the isomorphous structures of [M(H2O)(6)](C4H12N2)(HPO4)(2) with M = Co and Ni is based on an extended hydrogen bonded network between the three ionic building blocks.