Number of Atoms Packed in First Coordination Sphere around Metal Ion

Number of Atoms Packed in First Coordination Sphere AROUND Metal Ion 

Measurements of metal ion-liganding atom distances in the crystal structures of many inorganic salts led to the derivations of atomic or ionic radii for these atoms or ions. These radii are based on the idea that each atom or ion behaves as a solid sphere of defined size (Wasastjerna, 1923 Zachariasen, 1931). The shortest distance between adjacent ions of opposite sign is then presumed to be the sum of the radii of the respective ions the cation and the anion. 

However, since only values of rexpti are obtained, it is necessary to assume a value for the ionic radius of either r+ or r- in order to derive the ionic radius of the other. It is usual to assume a value of 1.40 A for the radius of the and 1.94 A for the radius of CP (Pauling, 1948) because these are half the minimum anion-anion distances found in crystal structures. Values for ionic radii (Shannon and Prewitt, 1969 Shannon, 1976 Brown, 1988) are listed in Table V for a coordination number of 6 around the metal atoms. Thus, values of radii are hypothetical, based on the idea of an additivity rule and a few initial assumptions on anion size. 

Effective Cation Radii of Some Metal Ions and Their Average Observed Coordination Numbers for Different Ligands in Structures of Small Molecule  

The coordination number of a cation depends on the number of anions or ligand atoms that can be fit around it in three dimensions (Fig. 2). In the hard-sphere model the coordination number is determined by the ratio of the radius of the cation to that of the anion 

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