The stereochemistry of binding of ligands by divalent metals and the propensity of these ligands to be water were investigated. Structural data of crystal structures in the Cambridge Structural Database (CSD) that contain divalent magnesium were examined with respect to the coordination 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 chelating groups; this implies that the magnesium ion often prefers to bind directly to water rather than to anions. In the absence of a polycyclic complexing agent such as porphyrins or crown ethers, the coordination number of magnesium in crystal structures tends to be six. The orientation of water molecules around a magnesium ion is found to be such as to bring the Mg2+...O-H angle as near as possible to 120-127 degrees (Mg...H near 2.6-2.7 Angstrom). Ab initio molecular orbital studies of the structures of hydrated Mg2+ ions With UP to seven water molecules partitioned between the first and second coordination shells were also carried out. The lowest energy configuration of hydrated Mg2+ has six water molecules packed into the inner coordination shell, although 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 configuration with seven water molecules arranged in the inner coordination shell could be found; the stable structure has six water molecules in the first shell and one in the second coordination shell, hydrogen bonded to two water molecules in the first shell. There appears to be less covalency in Mg2+...O than in Be2+...O interactions, water molecules are less affected by the presence of Mg2+ than by that of Be2+, and hydrogen bonding between the first and second coordination shell is weaker for magnesium than for beryllium complexes.