Thallium , electron

The compound Tl2Tc3 is a p-type semiconductor in which the thallium atoms are located in channels between puckered Te layers. A ° T1 and jvjmr study of this material has revealed that significant indirect exchange coupling occurs between the nuclei by overlap of the thallium electron wave functions, mainly across the Te atoms. Good agreement was found between the NMR conclusions and the calculated electronic stmcture and density of states in this semiconductor (Panich and Doert 2(XX)). 

Thallium 0.0005 0.002 Hair loss changes in blood kidney, intestine, or liver problems Leaching from ore-processing sites discharge from electronics, glass, and pharmaceutical plants. 

In the phosphor-photoelectric detector used as just described, the x-ray quanta strike the phosphor at a rate so great that the quanta of visible light are never resolved they are integrated into a beam of visible light the intensity of which is measured by the multiplier phototube. In the scintillation counters usual in analytical chemistry, on the other hand, individual x-ray quanta can be absorbed by a single crystal highly transparent to light (for example, an alkali halide crystal with thallium as activator), and the resultant visible scintillations can produce an output pulse of electrons from the multiplier phototube. The pulses can be counted as were the pulses-from the proportional counter. 

Group 13/III is the first group of the p block. Its members have an ns np1 electron configuration (Table 14.5), and so we expect a maximum oxidation number of +3. The oxidation numbers of B and A1 are +3 in almost all their compounds. However, the heavier elements in the group are more likely to keep their s-electrons (the inert-pair effect, Section 1.19) so the oxidation number +1 becomes increasingly important down the group, and thallium(I) compounds are as common as... 

Mercuration of aromatic compounds can be accomplished with mercuric salts, most often Hg(OAc)2 ° to give ArHgOAc. This is ordinary electrophilic aromatic substitution and takes place by the arenium ion mechanism (p. 675). ° Aromatic compounds can also be converted to arylthallium bis(trifluoroacetates), ArTl(OOCCF3)2, by treatment with thallium(III) trifluoroacetate in trifluoroace-tic acid. ° These arylthallium compounds can be converted to phenols, aryl iodides or fluorides (12-28), aryl cyanides (12-31), aryl nitro compounds, or aryl esters (12-30). The mechanism of thallation appears to be complex, with electrophilic and electron-transfer mechanisms both taking place. 

Metallation and oxymetallation reactions have been observed with the salts of only a few metals, namely mercury(II) (66, 67), thallium(III) (66,67), lead(IV) (66, 67), palladium(II) (100), gold(III) (63), and platinum-(II) (29). These facts correlate well with what Chatt (1) has termed class b, and Pearson (130) has called "soft acid character. Soft acids are characterized by low charge, large size, and, often, d electrons in their outer shell. No class b metal is known, in fact, which contains fewer than five d... 

Tl(III) < Pb(IV), and this conclusion has been confirmed recently with reference to the oxythallation of olefins 124) and the cleavage of cyclopropanes 127). It is also predictable that oxidations of unsaturated systems by Tl(III) will exhibit characteristics commonly associated with analogous oxidations by Hg(II) and Pb(IV). There is, however, one important difference between Pb(IV) and Tl(III) redox reactions, namely that in the latter case reduction of the metal ion is believed to proceed only by a direct two-electron transfer mechanism (70). Thallium(II) has been detected by y-irradiation 10), pulse radiolysis 17, 107), and flash photolysis 144a) studies, butis completely unstable with respect to Tl(III) and T1(I) the rate constant for the process 2T1(II) Tl(III) + T1(I), 2.3 x 10 liter mole sec , is in fact close to diffusion control of the reaction 17). 

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]. 

Colloids of more electronegative metals such as cadmium and thallium also act as catalysts for the reduction of water. In the colloidal solution of such a metal, an appreciable concentration of metal ions is present. The transferred electrons are first used to reduce the metal ions, thus bringing the Fermi potential of the colloidal particles to more negative values. After all the metal ions have been reduced, excess electrons are stored as in the case of silver. 

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. 

The discussion of the main group 3-5 and 3-6 compounds in the previous sections was limited to examples in which the group 3 element E is three-coordinate, so that an empty p-orbital on E is available for overlap with a lone pair on the group 5 or 6 atom. For the same reason, the discussion here will focus on those compounds with three-coordination at gallium, indium, or thallium. In the case of the transition metal derivatives, it is transition metal -electrons that are available to overlap with the empty p-orbital on E to form the potential ir-bond, as illustrated in Fig. 26. 

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