Coordination compounds Jahn-Teller distortion

The versatile binding modes of the Cu2+ ion with coordination number from four to six due to Jahn-Teller distortion is one of the important reasons for the diverse structures of the Cu-Ln amino acid complexes. In contrast, other transition metal ions prefer the octahedral mode. For the divalent ions Co2+, Ni2+, and Zn2+, only two distinct structures were observed one is a heptanuclear octahedral [LnM6] cluster compound, and the other is also heptanuclear but with a trigonal-prismatic structure. 

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. 

La2Cu04 has a structure and crystal chemistry virtually identical to that of La2Ni04 with a couple of important exceptions. Firstly, all octahedrally coordinated Cu compounds show a spontaneous electronic distortion (the Jahn-Teller distortion described in Section 8.3.1) by which the two axial bonds become longer and the four equatorial bonds become shorter. The distortion observed in La2Cu04 is usually attributed to this effect, but the observation of the same distortion in La2Ni04 shows that the driving force in both compounds... 

Transition metal difluorides are known mainly for first transition series elements, with palladium and silver difluorides from the second series, and no examples from the third. All these compounds have either the rutile structure, or, for chromium, copper, and silver, a distorted variant, which can be correlated with a Jahn-Teller distortion of the octahedral coordination of the ions. This rutile structure type is associated with smaller cations and, for comparison, although zinc difluoride has the same rutile structure, cadmium and mercury difluorides have the cubic fluorite structure with eight coordination of the cations (12). 

Fig. 39. The fraction ij of Mn sites without local Jahn-Teller distortions at 10 K and the Curie temperature Tc of various Ro.7Ao.3Mn03 compounds vs. the 9-coordinated ionic radii r after Louca et al. (2001).

Transition metal ions most susceptible to large Jahn-Teller distortions in octahedral coordination in oxide structures are those with 3d4, 3d9 and low-spin 3(f configurations, in which one or three electrons occupy eg orbitals. Thus, the Cr2+ and Mn3+, Cu2+, and Ni3+ ions, respectively, are stabilized in distorted environments, with the result that compounds containing these cations are frequently distorted from type-structures. Conversely, these cations may be stabilized in distorted sites already existing in mineral structures. Examples include Cr2+ in olivine ( 8.6.4) and Mn3+ in epidote, andalusite and alkali amphiboles ( 4.4.2). These features are discussed further in chapter 6. 

The 3(f ions Cr2+ and Mn3+ and the 3d9 ion Cu2+ are subject to Jahn-Teller distortions. For example, CuO does not have the rock salt structure, but one with four close Cu 0 neighbours and two at longer distance similar tetragonally distorted coordination is found in most other simple compounds of Cr2+ and Cu2+. (Note that CrO is unknown). 

The equal valence rule or loop rule equation (3) is less rigorously obeyed, and does not apply to the enviromnents of atoms with electronically driven anisotropies arising from, for example, lone electron pairs see Lone Pair, Electronic Structure of Main-group Compounds) or Jahn Teller distortions see Jahn-Teller Effect, Copper Inorganic Coordination Chemistry). 

In 1991 we reported on the synthesis of P-silyl and -phosphanyl substituted silylidenephosphanes (phosphasilenes) 2, i.e., compounds of type IIA and III, which are accessible by thermally induced elimination of LiF from corresponding / -lithium-(fluorosilyl)phosphanides IV [5, 6] and possess a remarkable thermal stability (up to 110°C). In 1993 the first crystalline phosphasilene 5 [7], a compound of type IV, had been prepared and its structure was established by X-ray diffraction [8], whereas the Si=P compounds of type V were merely characterized by means of NMR spectroscopy [9]. Compound 5 possesses a relatively long Si=P bond length and a non-planar geometry around the A, , a -Si atom, which can be explained in terms of steric hindrance or a second-order Jahn-Teller distortion [10, 11]. In comparison, the Si=P bond in 6 [12] is significantly shorter and the Si atom is trigonal planar coordinated. 

The autunite-type compounds with Mg or divalent transition metal cations (Mn, Fe, Co, Ni, Cu, Zn, Cd) in their interlayers are listed in Table 14 (phosphates) and Table 15 (arsenates). In the known structures of these compounds, the divalent interlayer cations are all in sixfold coordination appearing as nearregular octahedra with the exception of Cu, which shows a typically Jahn-Teller distorted sixfold coordination environment appearing as tetragonal dipyramids, and whose structures will be discussed separately. Three states of hydration are observed among these (Mn, Fe, Co, Ni) structures the triclinic dodecahydrates, the monoclinic decahydrates and the triclinic octahydrates [136, 138]. With progressive dehydration, the interlayer spacing decreases, the sheets shift in relative position, and the interlayer octahedra shift positions relative to the sheets (Fig. 34). 

The structures of ulrichite, Cu[Ca(H20)2(U02)(P04)2](H20)2, and of the mixed-valence synthetic compound [U (U02)(P04)] also exhibit substitution for uranyl in the uranophane sheet-anion topology (Fig. 46). In ulrichite, CaOs polyhedra alternate with uranyl pentagonal bipyramids, and the sheets are connected by Jahn-Teller-distorted Cu-centered octahedra [177]. In the structure of [U" (U02)(P04)], both and are in sevenfold coordination to form pentagonal bipyramids. Dimers of polyhedra alternate with dimers of uranyl pentagonal bipyramids along the uranium phosphate chains that make up the sheet (Fig. 46). The polyhedra also share vertices with tetrahedra of adjacent sheets to form a framework structure [178]. 

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