Thallium electronic structure

In its covalent compounds thallium shows no reluctance to utilizing the two 6s electrons for bond formation. Indeed monoalkyl derivatives Tl(Alk) in which the valence group would be (2, 2) are not known, whereas trialkyls (valence group 6) are known and the most stable alkyl derivatives are the dialkyl halides such as [T1(CH3)2]I. These are ionic compounds-[T1(CH3)2] OH being a strong base—and in the Tl(Alk)2 ions the thallium atom has the same outer electronic structure as mercury in CH3-Hg-CH3, viz. (4). Accordingly the (CH3-T1-CH3) ion is linear, as shown by the determination of the crystal structure of Tl(CH3)2l. In molecules such as Tl(Alk)2A, where A represents a molecule of a /3-diketone, T1 apparently has the valence group (8). 

Indeed, around 1980, first experimental results on atomic parity violation have been reported, in particular measurements of the optical activity of bismuth, thallium and lead vapours as well as measurements of an induced electric dipole (El) amplitude to a highly forbidden magnetic dipole transition (Ml) in caesium. These experiments have nowadays reached very high resolution so that even effects from the nuclear anapole moment, which results from weak interactions within the nucleus, have been observed in caesium. The electronic structure calculations for caesium are progressing to a sub-percent accuracy for atomic parity violating effects and the reader is referred to chapter 9 of the first part of this book [12]. 

Thallium (I) azide has the body-centered tetragonal structure of potassium, rubidium, and cesium azides (see Chapter 3), but it differs in being an easily initiated explosive, as well as having its fundamental optical absorption at lower energy (3.6 eV vs. approximately 8 eVfor KN3, RbNa, andCsNa see Chapter 5). It is of interest to investigate the effect of this difference in electronic structure on the ESR spectra of defects in the four materials having the same crystal structure. 

Since especially diatomics built from atoms with large nuclear charge number Z and an open p shell are significantly affected by scalar and spin-orbit effects, molecules containing thallium became perfect test cases for relativistic electronic structure methods. Some representative results for TIH are collected in a later section. 

The classical view of the lone pair is that, after mixing of the s and p orbitals on the heavy metal cation, the lone pair occupies an inert orbital in the ligand sphere [6]. This pair of electrons is considered chemically inert but stereochemi-cally active [7]. However, this implies that the lone pair would always and in any (chemical) environment be stereochemically active, which is not the case. For example, TIF [8] adopts a structure, which can be considered as a NaCl type of structure which is distorted by a stereochemically active lone pair on thallium. In contrast TlCl [9] and TlBr [10] adopt the undistorted CsCl type of structure at ambient temperature, and at lower temperatures the (again undistorted) NaCl type of structure. The structure of PbO [11] is clearly characterized by the stereochemically active lone pair. In all the other 1 1 compounds of lead with... 

Thallous halides offer a unique possibility of studying the stereochemistry of the (chemically) inert electron pair, since their structures and their pressure and temperature-dependent phase transitions have been well established. Thallium (1) fluoride under ambient conditions, adopts an orthorhombic structure in the space group Pbcm which can be regarded as a distorted rocksalt structure (Fig. 2.4). In contrast to TIF, the thallium halides with heavier halogens, TlCl, TlBr and Til, adopt the highly symmetric cubic CsCl structure type under ambient conditions [46]. Both TlCl and TlBr, at lower temperatures, undergo phase transitions to the NaCl type of structure [47]. 

KT1 does not have the NaTl structure because the K+ ions are too large to fit into the interstices of the diamond-like Tl- framework. It is a cluster compound K6T16 with distorted octahedral Tig- ions. A Tig- ion could be formulated as an electron precise octahedral cluster, with 24 skeleton electrons and four 2c2e bonds per octahedron vertex. The thallium atoms then would have no lone electron pairs, the outside of the octahedron would have nearly no valence electron density, and there would be no reason for the distortion of the octahedron. Taken as a closo cluster with one lone electron pair per T1 atom, it should have two more electrons. If we assume bonding as in the B6Hg- ion (Fig. 13.11), but occupy the t2g orbitals with only four instead of six electrons, we can understand the observed compression of the octahedra as a Jahn-Teller distortion. Clusters of this kind, that have less electrons than expected according to the Wade rules, are known with gallium, indium and thallium. They are called hypoelectronic clusters their skeleton electron numbers often are 2n or 2n — 4. 

Another contribution is represented by an investigation of a cubic thallium cluster phase of the Bergmann type Na13(TlA.Cdi A.)27 (0.24 < x <0.33) (Li and Corbett 2004). For this phase too the body centred cubic structure (space group Im 3, a = 1587-1599 pm) may be described in terms of multiple endo-hedral concentric shells of atoms around the cell positions 0, 0, 0, and 14,14,14. The subsequent shells in every unit are an icosahedron (formed by mixed Cd-Tl atoms), a pentagonal dodecahedron (20 Na atoms), a larger icosahedron (12 Cd atoms) these are surrounded by a truncated icosahedron (60 mixed Cd-Tl atoms) and then by a 24 vertices Na polyhedron. Every atom in the last two shells is shared with those of like shells in adjacent units. A view of the unit cell is shown in Fig. 4.38. According to Li and Corbett (2004), it may be described as an electron-poor Zintl phase. A systematic description of condensed metal clusters was reported by Simon (1981). 

The electron microscopy studies of the superconductive cuprates show that the different families differ from each other by the nature of their defect chemistry, in spite of their great structural similarities. For example, the La2Cu04-type oxides and the bismuth cuprates rarely exhibit extended defects, contrary to YBa2Cu307 and to the thallium cuprates. The latter compounds are characterized by quite different phenomena. 

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