Technetium electronic structure

Although there are a lot of publications on the chemistry of technetium [2-4] and transition-metal clusters [1,5-8], the chemistry of technetium clusters was insufficiently studied until the early eighties [1,2]. Nevertheless, the available scanty data on the compounds with Tc-Tc bonds inspired hope that interesting results would be obtained in the chemistry of technetium in general, in radiochemistry, and in the chemistry of transition-metal cluster compounds. The anticipated results were actually obtained [9-15] and the conclusion was drawn that technetium had a number of anomalous cluster-forming properties [9]. This review looks at the detailed studies of these properties and their interpretation in terms of electronic structure theory. 

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. 

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. 

In the process of their reduction, technetium ions unite to form increasingly large clusters. Fig. 1 shows the molecular structures of these clusters, Fig. 6 shows the results of the calculation of their electronic structure in terms of the EHT method [113]. 

Fig. 6. Results of the calculation of the electronic structure of technetium oxo- and halogenocom-plexes using EHT method [13, 113]...

Because magnetic, spectroscopic and other physicochemical properties of technetium clusters have been discussed in detail in our earlier reviews [9-15,77,103, 108,113,127-129], primary attention is given in this paper to a discussion of the electronic structure of technetium acido-clusters. 

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,... 

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. 

Electronic Structure of Polynuclear Technetium Clusters 5.3.1 Trigonal-Prismatic Clusters... 

The electronic structure of trigonal-prismatic technetium clusters was discussed in [142]. The main results obtained in this work are considered below. 

We next consider the participation of metallic outer ns(np) atomic orbitals in the formation of M-M bonds for close neighbors of technetium in the Periodic Table Mo, Re, and Ru. Fig. 9 presents calculated results of the electronic structures of MJ+ dimers. For Ru, the 2a m molecular orbital obviously has a significantly higher energy than the molecular orbital, and it is therefore always... 

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