Technetium binuclear complexes

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 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]. 

Literature data are available on the electronic structures of two more binuclear technetium complexes [(NHjLlOHLTcf/i-O TcfOH NHj ] (a hypothetical complex with the structure and composition analogous to those of the ethylen-diamminetetra-acetate complex [54,55]) and Tc2(CO)10 (a binuclear complex with strong crystal field ligands [168,169]. We shall consider the results of these calculations in greater detail. 

In section 5.2.3 it is shown that in contrast to binuclear complexes of other elements [1,10,11] for technetium compound this statement is not true. 

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. 

Tc2[(CH3)3CCOO]4 C12. The first binuclear d4-d4 technetium complex was obtained by Cotton and Gage [45] by oxidative substitution of the chloride ions in the coordination sphere of [Tc2Cl8]3 by trimethylacetate ions. The substitu-... 

Fig. 4. Schematic presentation of nearest ligand surrounding the binuclear d3-d3 and d3-d2 technetium complexes (according to [11])...

Electronic Structure of Binuclear Technetium Complexes 5.2.1 Octachloroditechnetates... 

Calculations of the electronic structure of the complexes of this type were first carried out for [Tc2C18]3, using the semi-empirical EHT method [146], It was shown that a strong quadruple M-M bond is formed in both the d4-d4 binuclear transition-metal complexes and d4-d5 technetium complexes. The addition of an excess electron does not decrease the Tc-Tc bond strength,... 

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]. 

Thus, the investigations performed [58,77] showed that [Tc2(/t-CH3COO)4] [TCO4J2 combines the properties of two classes of technetium complexes those of pertechnetates and those of binuclear carboxylates with multiple M-M bonds. On the other hand, the anionic and cationic fragments of this compound are characterized by a mutual effect which affects their electronic and molecular structures. 

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