Jahn-Teller distortion, divalent copper

Solvation of the divalent copper ion requires some special remarks. In solid state hexa-coordinate Cu2+ shows Jahn-Teller distortion the metal-ligand bonds of four ligands in the equatorial plane are shorter... 

The Cupric, Cu2+ or Cu(II) State, 3d9 The most important and stable oxidation state for copper is divalent. There is a well-defined aqueous chemistry of the Cu2+ ion, which generates the familiar blue solution when complexed with water. A large number of copper coordination compounds exist and these have been studied extensively. A strong Jahn-Teller distortion is associated with the 3d9 electronic configuration of this ion. This implies that a regular tetrahedron or octahedron about the Cu2+ ion is never observed, except in the rare occurrence of a dynamic Jahn-Teller effect. The tetragonal distortion about an octahedron can lead to a square-planar coordination which is often observed in Cu(II) oxides. 

Surprisingly many spinel-type chalcogenides form with copper as an A element. In a ternary representative Cu should be divalent in order to satisfy the valence rules. The spinel structure, however, shows no Jahn-Teller distortions contrary to the oxides. Assuming the cation-anion bonds to be saturated, the metallic properties must be due either to a metallic de band of Cu2+ or to the simultaneous presence of T4+ and T3+. 

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