Thallium structural stability

Reactions of Me3P=CH2 with MeaMCl (M = Ga, In, or Tl) form the new dimethylmetaldimethylphosphonium-bis(methylides), [Me2MCH2PMe2CH2] . Dimers (n = 2) may be isolated, and these possess eight-membered ring structures (76). This structure stabilizes the inherently labile thallium(iii) derivative." ... 

Metal alkoxide complexes with related alcohols are apparently edge-sharing bioctahedral dimers, on the evidence of the X-ray structure of the isopropyl complex. Some stable mixed alkoxides of the type Q[M3(OR)g] (Q = Li, Na, K, NH4, Ca/2) have been reported they distill in vacuo without decomposition. The X-ray structures of several bimetallic alkoxides obtained byCaulton,QZr2[0()-Pr)]g,Q = Li[HO(/-Pr)], K(DME), Ba[0(t-Pr)], show a similar triangular structure with two /X3 and three /x-OR bridges. The thallium salt of composition Tl2Zr(O R)e obtained by the reaction shown in equation (18) has a distorted octahedral structure stabilized by six T1 F contacts. 

The effect of structure of the alkyl group on the stability of monoalkyl-thallium(III) compounds can best be understood by reference to the different mechanisms by which these compounds undergo decomposition. A number of authors have attributed the instability of monoalkylthallium(III) compounds to facile C—T1 bond heterolysis and formation of carbonium ions [Eq. (25)] (52, 66, 79). This explanation is, however, somewhat suspect in cases where primary carbonium ions would be involved and either the two-step sequence shown in Eqs. (26), (27), or the fully synchronous 8 2 displacement shown in Eq. (28), is more compatible with the known facts. Examination of the oxythallation reactions that have been described reveals that Eq. (27) [or, for concerted reactions, Eq. (28)] can be elaborated, and that five major types of decomposition can be recognized for RTlXj compounds. These are outlined in Scheme 8, where Y, the nucleophile... 

Thallium may be described as a relativistic alkali metal the downshift in energy of the 65 orbital, due to a combination of relativity and shell structure effect, favours the oxidation state I over III (see 4.2.22). The stability of the oxidation state +1... 

For A, the nitrogen coordination is invariably planar because of a low to zero inversion barrier which facilitates possible overlap of the M and N p-orbitals. The n-overlap in B is facilitated by a linear geometry. However, a further consideration in the case of B is the increasing stability of the lower oxidation state of the metal when descending the group. This effect is most prominent in thallium and it implies an increasing stability of the monovalent unit R-M which may lead to destabilization or distortion of structure B. 

The atoms of the chemical elements, are, as I have already said, extremely complex, but their structure is not yet completely understood. To some part of each kind of atom its chemical properties and its spectrum are probably due. It is conceivable that this part may be the earliest to form, with its surrounding rings or envelopes at first not quite adjusted to permanent stability. With the final adjustment the isotopes as such should disappear, and the normal element be completed. This is speculation, and its legitimacy remains to be established. A careful comparison of the spectra of the elements from thallium up to uranium might furnish some evidence as to its validity. The spectrum of uranium, for example, may contain lines which really belong to some of its derivatives. 

For the Tlli ion,13 the stability of the iodide in contact with Tl111 is a result of the stability of the ion, since T1I3 is itself unstable relative to Tl F). Thallium also forms the ion T Clj, which has the confacial bioctahedral structure. 

A detailed, multimethod study of hydrated Tl(III) cyanide species in aqueous solution reveals that Tl(III) forms very strong complexes with cyanide ions (even stronger than halide-Tl(III) interactions)." " Formation of a series of Tl(III) complexes T1(CN) n= -4t) has been established, and the solution structures and stability constants were reported. The mono- and dicyano complexes [Tl(CN)(OH2)5] and [Tl(CN)2(OH2)4] show six-coordinate thallium centers, whereas Tl(CN)3(OH2) and [T1(CN)4] have four-coordinate T1(III) ions. 

The rates of alkaline hydrolysis of the half-esters, potassium ethyl oxalate, malonate, adipate, and sebacate were studied in the presence of potassium, sodium, lithium. thallium(I), calcium(II), barium(II), and hexamminecobalt(III) ions (106). On the basis of the results obtained, chelate formation between the metal ions and the transition state of the substrate was postulated. In these chelate structures (structures XXXVIII), formally similar to those postulated in the hydrolysis of a-amino esters (26), the metal ion facilitates the attack by the hydroxide ion by positioning it in a suitable manner. The rate of hydrolysis of the oxalate half-ester is greater than that of the malonate, which in turn is greater than that of the adipate. This is in the expected order of the stability of the metal chelates. The order for the rate of hydrolysis of the ethyl oxalate and ethyl malonate is Ca2+ Ba2+ > [Co(NH3)6]3+ > T1+. The hexamminecobalt(III) ion seems to be less effective than expected, since it is too large to satisfy the steric requirements of the chelate structures. The alkali metals were found to have marked negative specific salt effects on the rates of reaction of the adipate and sebacate, but only a small negative salt effect on the hydrolysis of potassium ethyl malonate. 

The crystal structures of the three heavier thallium(I) halides have been established and lattice energies calculated the inert pair of T1+ is apparently insignificant in terms of the stability of the lattice. The enthalpy of hydration of the Tl ion was also derived. Gas phase (TlBr, Til) and matrix isolation studies (TlCl, TlBr, Til) have shown that TlX and TI2X2 species are important.Complex formation in aqueous solution decreases in the order Cl > Br > I, and as with the fluorides, the double salts show no evidence of anionic complex formation by Tl Finally, it is important to note that TII3 is formulated as Tr(lT) from X-ray studies.Thermal decomposition yields TI3I4, whose structure has not been reported. 

Rare-earth elements stabilize different layered structures involving thallium, lead, and mercury, owing to their large size and also to their trivalent character allowing more oxygen to be incorporated. 

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