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= 4.9% and fi2 = 4.0% for 2218 observed reflections measured by counter techniques. The crystal structure of CaH2EHDP-H20 consists of an infinite array of H2EHDP2" and water molecules and Ca2+ ions linked together by hydrogen bonding and calcium coordination of oxygens from both H2EHDP2_ and water molecules. The H2EHDP2~anion exists in the crystal with one hydrogen on each phosphonyl group. The calcium ion has eight oxygen atoms in its primary coordination sphere: five from three symmetry-related H2EHDP2~ligands and three from water molecules. The CaH2EHDP structure represents the first reported example of a metal chelate ring involving the un-ionized hydroxyl group of a hydroxyphosphonic acid. A hypothetical model has been constructed for the formation and structure of Ca-EHDP polynuclear aggregates. In this model the importance of the presence of multiple metal-binding sites (i.e., more than two potentially stable chelating positions) is stressed. It is predicted that only those gem-diphosphonates, RiR2C(P03)2, where either Ri or R2 is a potential binding site should be capable of significant polynuclear aggregate formation.

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