Jahn-Teller effect complex oxides

In geochemistry, the Jahn-Teller effect is relevant for metals Fe and in octahedral complexes and for Cr + and Ni in tetrahedral complexes. Other transition ions (e.g., Cr and Co ) require unusual oxidation or low-spin conditions that can be reached only under extreme pressure. 

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. 

Although relatively few iron(IV) compounds have been characterized, transient Fe O species appear to be involved in many natural oxidation systems, especially in porphyrin and related complexes57 (see above). Several low-spin /t-nitridodiiron complexes, [Fe—N—Fe]n+, are known for n = 5,658 and they are prepared as shown in Fig. 17-E-6. Mixed oxides containing FeIV have been noted. In an octahedral environment, high-spin iron(IV) oxides show the typical distortion of the Fe06 moiety due to the Jahn-Teller effect.59... 

In a number of cases, the initial bond-length distribution was clearly not uni-modal, e.g. Figure A.2a. Where possible, such distributions were resolved into their unimodal components (as in Figure A.2c) on chemical or structural criteria. The case illustrated in Figure A.2, for Cu-Cl bonds, is one of the most spectacular examples, owing to the dramatic consequences of changes in oxidation state and coordination number and of Jahn-Teller effects on the structures of copper complexes. 

This effect is observed, for example, in inorganic complexes, where the metal ion is octahedral and the six ligands might be expected to occupy positions at equal distances at the corners of an octahedron, in an ideal structure. In the actual structure however, the octahedron may be distorted with four ligands in a square planar coordination and the remaining two ligands at opposite corners at longer distances. Many oxides exhibit Jahn-Teller distortions. 

The particular values and power dependence for the d-s mixing term are also not too critical although a certain threshold must be achieved. Tetragonally elongated Jahn-Teller distortions of d9 CuNe species (36) and the trigonal geometry of the oxidized copper center in Type 1 metalloproteins (37) can be achieved with an inverse sixth order power dependence and an associated a6 parameter of at least 300,000 kcal mol-1 A6. However, since eds also depends on symmetry—e.g., it makes no contribution for octahedral complexes—there are many systems where d-s mixing has a minimal effect. 

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