Tetragonal distortions of the octahedron

Fig. 9. (a) Qualitative MO diagram for octahedral TeYe , indicating vibronic coupling of ground state and first excited state (arrow, see text) (b) Splitting of the ttu state with tetragonal distortion of the octahedron (17),... 

Copper is markedly different in its reactivity compared to iron. In porphyrins copper is totally buried. Due to the rigid structure of the porphyrins, where no twisting of the ligands is possible no reactivity with oxygen is seen. Hence, copper porphyrin-complexes display no superoxide dismutase activity In biological systems copper is essentially coordinated to nitrogen or sulphur. In octahedral Jahn-Teller distorted Cu(II) complexes a tetragonal distortion of the octahedron, usually an extension... 

The low spin configuration was earlier found in the deep-coloured, tris chelated amino-tropon-iminates [Mn(LN)3] (for LN = 43). Black [Mn(LN)3], prepared from Mn2(CO)10 and LmH, had a magnetic moment of 3.12 BM whereas the brown-black [Mn(acac)2(LN)J and black [Mn-(acac)(LN)2] both were high spin species with / eff = 4.9BM (in CHC13).141 An X-ray structural analysis of the latter showed a tetragonal distortion of the [MnN204] octahedron.580... 

Fig. 2.4. Warped Mexican-hat type of potential surface for Jahn-Teller distortion of octahedral MXs molecule. The symmetry of the two-dimensional deformation space is 3 m, and displacements along the mirror iines correspond to distortions of the octahedron that preserve tetragonal symmetry (elongated or compressed octahedron)...

Other coordination polyhedra with D41, symmetry are the tetragonal bipyramid (CN = 6), which can be seen as a distortion of the octahedron, and the bicapped square prism (CN=10). The tetragonal bipyramid is not discussed here, because an angular distortion cannot lower the symmetry to D4. The distortion scheme of a bicapped square prism (BSP, D4h) to a bicapped square antiprism (BSAP, D4d) is analogue to the one described above. The PCEM expressions for the B and 5 parameters will be exactly the same as for the SP-SAP distortion scheme, since axial ligands do not have an influence on... 

In fact the vast majority of 6-coordinate complexes are indeed octahedral or distorted octahedral. In addition to the twist distortion just considered distortions can be of two other types trigonal and tetragonal distortions which mean compression or elongation along a threefold and a fourfold axis of the octahedron respectively (Fig. 19.8). 

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. 

Figure 6.55 Distortion of TiOg octahedron in the tetragonal BaTiOs (top) and possible orientations of the polar axis when an electric field is applied along the pseudo-cubic (001) direction of BaTi03 (middle). Polar axes are shown by arrows inside each cube. Phase transitions in BaTiOj accompanied by changes in (a) dielectric constant (b) spontaneous polarization (c) heat capacity and (d) lattice dimensions (bottom).

The tetragonal distortion most commonly involves an elongation of one C4 axis and, in the limit, two trans ligands are lost completely, leaving a square, 4-coordinate complex. The trigonal distortion transforms the octahedron into a trigonal antiprism. 

---