Electronic Configuration. Jahn-Teller Effect

Some peculiarities of the described structure types can not be accounted for by the ionic aspects discussed so far. One may assume, that such peculiarities are connected with the electronic configurations of the ions concerned 231). Effects that depend on anomahes of the ionic sizes for special electronic configurations (e.g. maxima for high-spin or minima for low-spin d , see f. i. Blasse (36) Chadwick and Sharpe (63)) will be neglected, however, as well as the interesting questions connected with the stability of valency states (see f.i. Sheldon (279)). 

A dominant influence of certain electronic configurations on the structure type is represented by the Jahn-Teller effect (173). It shall be discussed here only briefly. For further information the reader is referred to a review by Or gel and Dunitz (241) and as for the general base of ligand field theory to the books of Orgel (240) and Figgis (101). 

The lowering of symmetry in fluoride structures that contain Jahn-Teller ions instead of normal ones is explained this way. All cases of high-spin configurations d (Cr2+, Mn +) and (Cu2+) provide examples of distorted MeFa-octahedra. The overall distortion always leads to elongated octahedra, as accounted for theoretically by Opik and Pryce (234). 

The only example of a tetragonal compressed octahedron, theoretically predicted as well (212, 214), seemed to be represented by the 

The interelectronic repulsion brought about by the doubly occupied cfz2-orbital at the vertices of the octahedron removes ligands from there completely or to a great distance, whereas the unoccupied ij 2- 2-orbital favours 4 ligands in a near square arrangement. 

---

The ti eatment of the Jahn-Teller effect for more complicated cases is similar. The general conclusion is that the appearance of a linear term in the off-diagonal matrix elements H+- and H-+ leads always to an instability at the most symmetric configuration due to the fact that integrals of the type do not vanish there when the product < / > / has the same species as a nontotally symmetiic vibration (see Appendix E). If T is the species of the degenerate electronic wave functions, the species of will be that of T, ... 

The Jahn-Teller effect is always to be expected when degenerate orbitals are unevenly occupied with electrons. In fact, it is observed for the following electronic configurations ... 

Considering the influence of electronic configurations on crystal structures it may be asked, whether certain structure t5rpes are restricted to fluorine compounds of the transition elements. Apart from the structure types distorted by the Jahn-Teller effect such a limitation is not obvious at all. On the contrary quite a number of structure prototypes are represented by compounds of the main group elements. Bonding thus must be similar in both, main group and transition element fluorides, at least as for the factors that influence crystal structmes. 

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. 

A very similar situation is presented by the dynamic Jahn-Teller effect, in which a trapped electron that has distorted its surroundings hops between two configurations with a frequency given by (6) for a proof see Sturge (1967). 

In the structure, the copper (or chromium) atom has a greatly distorted octahedral coordination, with two bond distances much longer than the other four (difference = 0.38 A for Cu, 0.36 A for Cr) and small, but significant differences between the pairs of short distances. The major distortion is correlated with the d9, or high-spin d4, electronic configuration of Cu(II) and Cr(II), respectively, giving an uneven occupation of the eg orbitals—the Jahn-Teller effect. The differ-... 

The compound Cr2As should have the charge configuration Cr Oi As3-. Since the octahedral interstices have tetragonal (c/a >1) symmetry, the single eg electron at a Cru occupies a dti orbital oriented parallel to the c axis. This ordering increases the axial ratio (Jahn-Teller effect). With c/a 1.76, R2 = 2.53 A < =... 

As a consequence of the non-spherical symmetry of the copper(II) ion, d9 configuration, and of the influence of the Jahn-Teller, and pseudo Jahn-Teller effect on six-coordinate geometries, the stereochemistries of the copper(II) ion are characterized by non-rigid geometries (fluxional behaviour), and ranges of distorted geometries (Plasticity Effect). The latter may be connected by a series of Structural Pathways, which may be characterised by an Electronic Criterion of Stereochemistry for a related series of complexes, e.g. the [Cu(bipy)2X] [Y] complexes. 

---