Tetragonal distortion of octahedral complexes

The iron(Il) halide and pseudohalide coordination complexes formed with pyridine may be divided into two types. The first type consists of complexes with the general stoichiometry, Fe(pyridine)4X2, where X represents the halides and pseudohalides listed in Tables 3.4 and 3.5. They are found to be high spin monomeric tetragonally distorted trans octahedral complexes and show paramagnetic behaviour down to around... 

Tetragonal distortion from octahedral symmetry often occurs even when ail six ligands of a complex are the same Two L groups that are irans to each other are found to be either closer to or farther from ihe metal ion than are the other four ligands. A distortion of this type actually is favored by certain conditions described by the Jahn-Teller theorem. The theorem stales that for a nonlinear molecule in an electronically degenerate state, distortion must occur to lower the symmetry, remove the degeneracy, and lower the energy.47 We can determine which octahedral complexes... 

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 observed tetragonal distortion of an octahedral complex is accompanied by a change in symmetry (Oh to Z)4h) and a splitting of the Cg and t2g sets of orbitals (see Figure 21.10). Elongation of the complex (21.5) is accompanied by the stabilization of each d orbital that has a z component, while the dxy and d 2 orbitals are destabilized. 

We have now considered the distortion of octahedral structures caused by the presence of 0, 1, 2, 3, 4, 5 (unpaired), and Idd electrons. It should be elear that high-spin (f, (f, cf, and cf systems are similar to r/, ef, ef, and d systems, respectively. Six-coordinated eomplexes of ef metal ions exhibit distortions similar to those of d eomplexes. The most common examples are eopper(n) complexes. In [CufNHs), the tetragonal distortion is so marked that the square planar tetraammine eomplex results. Note, however, that solvent molecules occupy positions above and below the plane in solutions of eomplexes of this type these solvent moleeules are farther from the metal ion than are groups in the square plane. The distortion of symmetrieal struetures resulting fi om partially filled eleetronie energy levels (in this ease the d sublevel) are called Jahn-Teller distortions. 

One-electron energy levels associated with metal d orbitals in complexes of various symmetries (a) octahedral (b) tetrahedral (c) tetragonal distortion of an octahedron (d) trigonal distortion of an octahedron (e) trigonal distortion of a tetrahedron. 

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). 

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