Technetium complexes clusters

Among other widely used reducers for the synthesis of binuclear technetium complexes with M-M bonds, zinc in HC1 and H3P02 [1,11] are most important. These reducers, as a rule, are used for the synthesis of Re clusters of similar structure. 

From the results presented it follows that the driving force behind the growth of technetium clusters in the process of their reduction is a decrease in the total electron energy of the ions due to the formation of M-M bonds. In fact, as is shown in Fig. 6, if the M-M bonds were absent the total electron energy of technetium complexes would be considerably higher and the complex would be unstable. However, besides purely thermodynamic reasons leading to the cluster formation, there should also be kinetic possibilities for these processes to take place. This aspect of technetium cluster formation is partially considered below. 

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

This facile approach to the carbene chemistry of rhenium has not yet been investigated with technetium. Further reactions with the technetium cluster 44a have been performed in C6H6/HC1 to yield the compound [( 6H6)Tc(CO)3]+ (66) which previously had only been described for manganese and rhenium [81]. "Tc-NMR of the latter compound exhibits a resonance at -1983 ppm (relative to [Tc04]- ), and it therefore fits very well into the range proposed for Tc(I) complexes. 

Metal-metal (M-M) bonds, first noted in the early sixties, occur in several thousand transition-metal compounds [1]. Complex technetium compounds and compounds with M-M bonds (clusters) have been studied more extensively than many other classes of inorganic compounds. Increasing interest in technetium compounds is due to the practical uses of the "mTc isotope, which ranks first among radioactive isotopes used in nuclear medicine diagnostics [2-4]. On the other hand, technetium clusters are an interesting object for theoretical studies, because until recently, they were the only compounds in which the presence of these anomalous chemical bonds was thought possible. 

In compliance with the nuclearity principle, polynuclear clusters are subdivided into a number of other subgroups, e.g. hexanuclear, octanuclear, etc. The binuclear clusters of technetium may be classified according to the electronic structure of their Tc-Tc2 bonds. Then, the d4-d4 complexes with quadruple M-M bonds are the father of all binuclear complexes with Tc-Tc bonds. The addition or removal of electrons from Tc-Tc bonds [1,11] should result in a decrease in the formal multiplicity of M-M bonds. Thus, for instance, the formal multiplicity of Tc-Tc bonds of d3-d3 and d5-d5 binuclear complexes equals3 3, that of d4 d5 and d4-d3 complexes equals 3.5, etc. 

The specific features of the cluster compounds of technetium are such, that practically each new compound must be studied using single crystal X-ray structural analysis, because their complex structures do not allow the interpretation of the results from other physico-chemical methods of investigation. Therefore, the synthesis of single crystals suitable for X-ray structural analysis is the main and most laborious chemical task. 

The paper electrophoresis experiments carried out to study the mobility of polynuclear technetium clusters in aqueous solutions of HX of varying acidity, as a mobile phase, showed that these clusters were also characterized by reversible reactions such as (5) without leading to destruction of M-M bonds. On the other hand, an autoclave recrystallization of the polynuclear clusters at 200-220°C in an atmosphere of argon from concentrated solutions of HX led to a partial destruction of M-M bonds and the formation of binuclear complexes [Tc2X8]3 and [Tc,X6]2. This indirectly shows that reactions (6) and (7), leading to the destruction of M-M bonds, are likely in solutions of polynuclear clusters [15]. 

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]. 

Isomer shift data, Fe,S4 clusters, 38 20, 50 Isomorphic substitution, 39 179, 186 p-Isonicotinamide complexes, osmium, 37 307 p-lsonicotinamidepoly(proline) complexes, osmium, 37 307 Isonitrile complexes osmium, 37 245 technetium(I), 41 13-14 technetium(II), 41 31 technetium(IIl), 41 45 Isopolymolybdates, 19 239ff 19 265-280 crystallization from aqueous solution, 19 265-269... 

Technetium(II) complexes are paramagnetic with the d5 low-spin configuration. A characteristic feature is the considerable number of mixed-valence halide clusters containing Tc in oxidation states of +1.5 to + 3. This area has been reviewed (42). For convenience, all complexes, except those of [Tc2]6+, are treated together here. EPR spectroscopy is particularly useful in both the detection of species in this oxidation state and the study of exchange reactions in solution. The nuclear spin of "Tc (1 = f) results in spectra of 10 lines with superimposed hyperfine splitting. The d5 low-spin system is treated as a d1 system in the hole formalism (40). 

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