Stabilization of technetium

In addition to research on the structure and other physicochemical properties in the solid state and solution, research into the thermal stability of technetium... 

Love WG, Amos N, Williams BD, Kellaway IW. Effect of liposome surface charge on the stability of technetium ( Tc) radiolabelled liposomes. J Micro-encapsul 1989 6 105. 

T.M. Gilliam, R.D. Spence, W.D. Bostick, and J.L. Shoemaker, Solidification/stabilization of technetium in cement-based grouts, Proc. 2nd Annual Gulf Coast Haz. Substance Research Center Symp., Solidification/ Stabilization Mechanisms and Applications, Beaumont, 7X February 15—16, 1990. 

The chemistry of technetium(II) and rhenium(II) is meagre and mainly confined to arsine and phosphine complexes. The best known of these are [MCl2(diars)2], obtained by reduction with hypophosphite and Sn respectively from the corresponding Tc and Re complexes, and in which the low oxidation state is presumably stabilized by n donation to the ligands. This oxidation state, however, is really best typified by manganese for which it is the most thoroughly studied and, in aqueous solution, by far the most... 

Diamine chelate complexes are more stable than the monodentate amine heterocycles and, therefore, can be studied under physiological conditions. The imidazole complexes are unstable in aqueous solution and decompose rapidly to technetium oxide hydrate. Six-membered ring chelates are significantly less stable than five-membered ones. Lesser flexibility of the ligand, such as 1.2-diamino-cyclohexane, parallels somewhat lower stability of the complex [53] ... 

Most of the substitution reactions with the homoleptic Tc(I) isocyanide complexes presented in the preceding section had to be performed at elevated temperatures and were often characterized by low yield. The reason for this behaviour is the exceptionally high kinetic and thermodynamic stability of this class of compounds. From this point of view, 4a are not very convenient or flexible starting materials, although they are prepared directly from 3a in quantitative yield. The exceptionally high kinetic and thermodynamic stability is mirrored by the fact that it was not possible to substitute more than two isocyanides under any conditions. On the other hand, oxidation to seven-coordinated Tc(III) complexes occurs very readily. Technetium compounds of this type, which are not expected to be very inert, could open up a wide variety of new compounds, but this particular field has not been investigated very thoroughly. A more convenient pathway to mixed isocyanide complexes that starts with carbonyl complexes of technetium will be described in Sects. 2.3 and 3.2. 

It should be noted that the above classification system of technetium cluster compounds is not the only possible one. In section 4 another classification is described, which is based on thermal stability and the mechanism of thermal decomposition. Section 2.2 is concerned with the classification based on methods of synthesizing cluster compounds. The classifications based on specific properties of clusters do not at all belittle the advantages of the basic structural classification they broaden the field of application of the latter, because for a better understanding and explanation of any chemical, physico-chemical and physical properties it is necessary to deal directly or indirectly with the molecular and/or electronic structures of the clusters. 

These compounds constitute the most numerous groups of technetium clusters which have been investigated (Table 1, compounds 8-16). In our view this is due to the greater stability of the d4-d5 clusters compared with the d4-d4 clusters and also the fact that compounds with the [Tc2Cl8]3 anions were obtained initially for technetium [22,24,42,43] and were subsequently employed to synthesize new technetium clusters [11,60-62],... 

Thus partial evaporation of HC1 during the synthesis of [Tc2C18]3 by the method of Eakins et al. [24] and by the autoclave method [22,42,43] creates favorable conditions (from the standpoint of the acidity of the solution and the concentration of technetium ions in it) for the formation of the octachloro-ditechnetates (+2.5), since, on the one hand, the rate of reduction of the technetium(IV) ions increases owing to their hydrolysis [42] and, on the other hand, the stability of the hydrolyzed cluster ions formed increases in relation to the reactions involving disproportionation and oxidation by atmospheric oxygen [9,52,80,87]. We may note that under the conditions of more pronounced hydrolysis, the rate of reduction of technetium(IV) increases so much that the formation of metallic technetium becomes possible. 

Such a classification of technetium cluster compounds, in our opinion, reflects the relationship between the thermal stability and structure of the clusters quite well. Moreover, on the basis of this classification it is easier to follow the mechanism of the main thermochemical transitions of technetium clusters, such as (1) dehydration (2) disproportionation and related processes occurring without changes or with only small changes in mass (3) one-stage processes of thermolysis. We shall now consider these main mechanisms of the thermochemical reactions of technetium clusters in greater detail. 

Table 3. Thermal stability of the technetium cluster compounds... 

Whatever the advantages of the SCF calculations with the Cl s, they do not always give good results, as is the case for M2 molecules with quintuple and sextuple M-M bonds [135,156], Nevertheless, some deviations from the simple MO scheme can be explained using simpler qualitative arguments. For example, the increased ability of technetium to form d4-d5 complexes and their greater stability in comparison to that of the d4-d4 complexes are explained on the basis of a model of the electrostatic repulsion of M atoms with like charges in a binuclear cluster [10,90,150] or on the basis of different diffusivities of cr, o, n, 5 and S metallic MO s of the clusters [63,141,157]. 

To conclude this section, we consider one more probable way for the stabilization of octahedral clusters, which, as was shown in the previous section, is characteristic of binuclear, trigonal-prismatic and, obviously, all types of technetium acido-clusters [10]. According to [12,77], all technetium clusters with an odd number of metallic electrons have shorter multiple M-M bonds than those in analogous structures, but with an even number of metallic electrons. In our opinion [10, 15], this effect of an odd number of metallic electrons is essentially analogous to the effect of the increase in M-M bond... 

Many compounds of technetium and rhenium are of analogous composition and of corresponding physical and chemical properties. Because of the very similar ionic radii, isotypic crystal structure formation of analogous compounds could often be observed. Technetium remarkably differs from manganese by the high stability of pertechnetate compared with permanganate. Moreover, divalent technetium does not exist as a hydrated ion but only as a stabilized complex. 

The [Tc=N] core is second only to the [Tc=0] core in its importance for the coordination chemistry of technetium in the context of radiopharmaceutical applications. The nitrido ligand N is isoelectronic with the 0x0 o ligand and with the imido R—ligand. The highly negative charge makes the nitrido ligand a very powerful rr-donor, well able to stabilize the... 

By far, the largest number of structurally characterized rhenium complexes contain the metal in the oxidation state +5 . This can be attributed to the high stability of the rhenium(V) oxo, nitrido, and imido cores with a great variety of ligand systems, but is doubtlessly also related to the fact that rhenium complexes are frequently used as nonradioactive model compounds for the development of technetium radiopharmaceuticals. The dominance of 0 , and NR ligands can be... 

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