Table 1: Coordination geometry, ionic radii, and hydration energies.

Ion	No. waters in inner shell	Coordination geometry	Half-life inner shell	Radius (Å)	ΔH° (kJ/mol)
Li+	4	-	10 ps	0.60	-564
Na+	4	-	10 ps	0.95	-438
K+	8	twisted cube	10 ps	1.33	-355
Mg2+	6	octahedral	0.1 ms	0.65	-1990
Cl-	6	octahedral	10 ps	1.81	-342

Comments:

Magnesium is unique among biological cations. Its volume change from hydrated cation to atomic ion is over an order of magnitude larger than that of any other biological cation. The hydration energy reflects the strong binding of water molecules, as does the slower rate at which water molecules depart from the inner shell (over four orders of magnitude slower than that for the other ions in the table). Whether you regard the bonding as charge-dipole interactions or partial covalent bonds isn't important. What is important is the octahedral configuration of the inner shell.

Because of these unique properties, magnesium ions have a low affinity for proteins. This is important, because it leaves Mg²⁺ available for binding phosphate groups in ATP, RNA, etc., either as the fully hydrated species or as a partially dehydrated species with negatively charged oxygen atoms of the phosphate groups replacing water from the inner hydration shell.

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