Aqua ions cadmium

DIVALENT ZINC AND CADMIUM COMPOUNDS 15-7 Aqua Ions, Oxides, and Hydroxides... 

Zinc and cadmium form numerous water soluble salts, for example, the nitrates, sulfates, sulfites, perchlorates, and acetates, that contain the aqua ions, [M(H20)6]2+. These aqua ions are also present in aqueous solutions.9 Some salts of Zn2+ and Cd2+ are isomorphous with those of Mg2+. The [M(H20)6]2+ ions are slightly hydrolyzed in water ... 

There is a significant difference in the affinity of the two metal irais towards the complexation with halide ions, too. In a diluted aqueous solution of zinc(II) chloride the octahedral aqua ions [Zn(H20)e] are the predominating species, while a significant ratio of chloro complexes are formed with cadmium(II) under the same conditions. The stability constants of the complexes formed with bromide or iodide ions are even higher. The consideration of these differences is especially important during the selection of the appropriate counter ions to adjust the ionic strength for thermodynamic or electrochemical studies or even in the synthesis of cadmium(II) compounds. 

Cd(H(N7)ade)(H20) units are linked by H2-oxo bridges with tetradentate oxalate ligands. Curiously, Hade coordinates through the less basic heterocyclic donor atom, giving rise to the Cd-N3 bond (2.282 A), which is assisted by an (aqua) O-H—N9 H-bond (2.766 A, 147.7°). Thus, the tautomer H(N7)ade seems to be favored due to the involvement of the N7-H bond in the crystal packing. The compound [Cd(p3-S04)(p2-N7,N9-H(Nl)ade)] [8] craitains the tautomer H(Nl)ade, a situation consistent with the bidentate role of adenine and its basicity order (N9 > N1 > N7 > N3 > N6). The Cd-N9 bond (2.239 A) does not cooperate with an interligand H-bonding interaction, whereas the Cd-N7 bond (2.204 A) is reinforced by a N6-H -0(sulfate) interaction (2.940 A, 164.7°). In these polymers the cadmium ion adopts a six-coordination mode. 

The cadmium(ll) chelate of the tripodal tetradentate ligand tren also yielded the mixed-ligand complex cation of the salt [Cd(tren)(xan)]C104 H20 [29], where the metal ion is only five-coordinated and the anionic xan ligand forms the Cd-N7 bond (2.224 A) reinforced by the intramolecular interligand H-bond (tren)N-H" 06 (2.889 A, 117.6°). In the compotmd traui-[Cd(theo)2(H20)4]-2DMF [30], each theophilinate(l-) anion binds the cadmium(ll) center by the Cd-N7 bond (2.300 A) in cooperation with an intramolecular H-bond (aqua)0-H - 06 (2.689 A, 158.5°). 

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