<Li> Solvate or hydrate isomerism--the isomers have the same composition but differ with respect to the number of molecules of solvent that serve as ligand vs simply occupying sites in the crystal . Examples: (Cr (H O)) Cl is violet colored, (CrCl (H O)) Cl H O is blue - green, and (CrCl (H O)) Cl 2H O is dark green . See water of crystallization . </Li> <Li> Linkage isomerism occurs with ambidentate ligands that can bind in more than one place . For example, NO is an ambidentate ligand: It can bind to a metal at either the N atom or an O atom . </Li> <Li> Coordination isomerism--this occurs when both positive and negative ions of a salt are complex ions and the two isomers differ in the distribution of ligands between the cation and the anion . For example, (Co (NH)) (Cr (CN)) and (Cr (NH)) (Co (CN)). </Li> <P> Many of the properties of transition metal complexes are dictated by their electronic structures . The electronic structure can be described by a relatively ionic model that ascribes formal charges to the metals and ligands . This approach is the essence of crystal field theory (CFT). Crystal field theory, introduced by Hans Bethe in 1929, gives a quantum mechanically based attempt at understanding complexes . But crystal field theory treats all interactions in a complex as ionic and assumes that the ligands can be approximated by negative point charges . </P>

Coordination isomerism occurs when both the cation and anion are
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