Technetium structural data

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. 

The main difference between the structures of compound (IV) and compounds (I), (II), and (III) is that in compound (IV) half of the bridging and terminal bromine atoms, and all the axial bromine atoms, are substituted by iodine atoms, with partial substitution being statistical [75]. Thus, according to the X-ray diffraction data, the effective Tc-Tc distances are intermediate between analogous distances in bromide and iodide complexes of technetium(IV) (Table 1) [104,105]. We observed that the substitution of I for Br has almost no effect on the Tc-Tc distances. 

However, X-ray diffraction data show [68,69] that in K2[Tc2C16], the Tc-Tc bond distance is 0.1 A shorter than that in an analogous d4-d4 chloride technetium complex, although its main structural fragment [Tc2C18]4- has a staggered structural conformation. That is why a study of the electronic... 

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 contrast with the difluorides, the distribution of trifluorides extends to the third series of the transition metals, where iridium and gold trifluorides are fully characterized. In the second series, trifluorides are known for the elements from niobium to rhodium, with the exception of technetium, and in the first series, from titanium to cobalt. All the trifluorides have been characterized structurally, with earlier reports based on X-ray powder-diffraction data, since the compounds were not prepared in single-crystal form until more recently, when high-temperature, crystal-growth techniques became available. 

The catalytic metal most widely studied by NMR is platinum the hrst observation of oxide-supported Pt was made by Slichter and co-workers 20 years ago. Nearly all of the metal-NMR results in this review are concerned with this nucleus. Some data for Rh will be discussed also. Recently, Tc NMR spectra have been published of small Tc particles (average diameter 2.3 nm, but a rather wide size distribution) on alumina [70]. The spectra were taken between 120 and 400 K. While bulk technetium has the hep structure, these small particles are cubic, and their Tc shift (around 7400 ppm) is about 600 ppm larger than the isotropic part of the bulk shift. The linewidth varies with support material and method of preparation, but remains amazingly small (15-75 ppm). This linewidth/shift ratio of about 0.5% is much less than that found for small particles of rhodium or platinum and is comparable to that found for silver [71]. It is unlikely, however, that the linebroadening mechanisms in small particles of silver and of technetium are the same. 

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