COORDINATION OF WATER TO MAGNESIUM CATIONS

The stereochemistry of binding of ligands by divalent metals and the p ropensity of these ligands to be water were investigated. Structural d ata of crystal structures in the Cambridge Structural Database (CSD) t hat contain divalent magnesium were examined with respect to the coord ination of water molecules and other groups around a magnesium ion and any possible variability in the coordination number of magnesium. The analysis highlighted the stability of the hexaaquated magnesium ion, Mg[H2O](2+)(6), which is found in the presence of various anions or ch elating groups; this implies that the magnesium ion often prefers to b ind directly to water rather than to anions. In the absence of a polyc yclic complexing agent such as porphyrins or crown ethers, the coordin ation number of magnesium in crystal structures tends to be six. The o rientation of water molecules around a magnesium ion is found to be su ch as to bring the Mg2+...O-H angle as near as possible to 120-127 deg rees (Mg...H near 2.6-2.7 Angstrom). Ab initio molecular orbital studi es of the structures of hydrated Mg2+ ions With UP to seven water mole cules partitioned between the first and second coordination shells wer e also carried out. The lowest energy configuration of hydrated Mg2+ h as six water molecules packed into the inner coordination shell, altho ugh structures with five water molecules in the first shell and one in the second or four in the first shell and two in the second are less than 5 and 10 kcal/mol, respectively, higher in energy. No stable conf iguration with seven water molecules arranged in the inner coordinatio n shell could be found; the stable structure has six water molecules i n the first shell and one in the second coordination shell, hydrogen b onded to two water molecules in the first shell. There appears to be l ess covalency in Mg2+...O than in Be2+...O interactions, water molecul es are less affected by the presence of Mg2+ than by that of Be2+, and hydrogen bonding between the first and second coordination shell is w eaker for magnesium than for beryllium complexes.