Halide anion, hydration sphere

The hydration of anions is regarded as being electrostatic with additional hydrogen bonding. The number of water molecules in the primary hydration sphere of an anion depends upon the size, charge and nature of the species. Monatomic anions such as the halide ions are expected to have primary hydration spheres similar to those of monatomic cations. Many aqueous anions consist of a central ion in a... 

Anions have a much weaker tendency to coordinate water molecules than cations and precise values for hydration numbers are difficult to obtain from X-ray data. Although a hydration sphere around an anion is often included in the least-squares analysis of an intensity curve for a metal salt solution, meaningful values are usually obtained only for the bond lengths. Coordination numbers and rms variations are often kept constant at assumed values. For the halide ions the bond lengths found show no significant deviations from values in crystal structures. 

The strong hydration of halide ions in water is partly due to outer-sphere effects. Oxo anions in aqueous solution may complex with outer-sphere water molecules as in the hydrated chromate and permanganate ions ... 

If the inner sphere coordination is a highly dominant term in the lattice energy of salts of a metal ion then the crystal structures should be just the packing of complexes of a fixed M-dependent structure. For example metal hydrates of SO4- give compounds [M(H20)6]n+ SO4 n H20 for Mg2+, Ni2+, Al3+, Cr3+, Co3+, Mn2+ etc. and similar hydrates appear in halides, and salts of other strong acid oxy-anions, e.g. nitrates. Even when water molecules do not occupy all sites around M the octahedron is maintained [Exceptions such as Co(II) and Zn(II) giving M(0)4 polyhedra are easily explained on the basis of polarisation (ligand-field) terms but these are still central field effects, i.e. 

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