Group technetium

The congeners of manganese are very rare. The middle element of the group, technetium (Z = 43), has not been isolated from mineral sources, and there is good reason to suppose that detectable quantities do not occur naturally (Chap. 27, Exercise 7). Weighable amounts of this element and its compounds have been made from molybdenum by nuclear displacement and from uranium by nuclear fission (p. 474). Rhenium (Z =5 75) occurs naturally but in only tiny amounts. It has been estimated that there is one atom of rhenium present in the earth s crust for each two billion (2 X 109) atoms of silicon. Rhenium was discovered in 1925, technetium in 1937. 

Physical Properties. Molybdenum has many unique properties, leading to its importance as a refractory metal (see Refractories). Molybdenum, atomic no. 42, is in Group 6 (VIB) of the Periodic Table between chromium and tungsten vertically and niobium and technetium horizontally. It has a silvery gray appearance. The most stable valence states are +6, +4, and 0 lower, less stable valence states are +5, +3, and +2. 

Certain neutral technetium complexes can be used to image cerebral perfusion (Fig. 4). Those in Figure 4a and 4b have been approved for clinical use. Two other complexes (Fig. 4c and 4d) were tested in early clinical trials, but were not developed further. An effective cerebral perfusion agent must first cross the blood brain barrier and then be retained for the period necessary for image acquisition. Tc-bicisate is retained owing to a stereospecific hydrolysis in brain tissue of one of the ester groups to form the anionic complex TcO(ECD) , which does not cross the barrier. This mechanism of retention is termed metaboHc trapping. 

As a general rule, elements in the second and third transition series have similar chemical properties. In contrast, the properties of the first member of the series are often different. This pattern of behavior is seen in Group 7 (VIIB). The properties of rhenium and technetium differ considerably from those of manganese. 

Some of the important properties of Group 7 elements are summarized in Table 24.1. Technetium is an artificial element, so its atomic weight depends on which isotope has been produced. The atomic weights of Mn and Re, however, are known with considerable accuracy. In the case of... 

Manganese, Technetium and Rhenium Table 24.6 Oxohalides of Group 7... 

Tris(dioxime) complexes that are capped with a boryl group BR at only one end of the molecule are also known, e.g., with technetium(III) [227, 228] and rhenium(III) [229]. 

The tracer level Tc,nHA complex can be suspended as a colloid in an aqueous solution. Spedation of a similar complex, Am111 HA, in the Gorleben groundwater was performed by laser photoacoustic spectroscopy (LPAS) [33]. Considering its molecular size, however, migration of the complex is expected to be very slow when it is present in a solid phase. Similar LPAS studies of a technetium complex towards chemical spedation have been tried by our group in Sendai [36,37]. 

Goldberg [44], surprisingly high concentrations were detected for the former 16, 000 Bq/kg, for the latter 43,000-46,000 Bq/kg. All these data indicate that technetium should be carefully recovered in reprocessing plants. The systematic recovery of technetium together with elements of the platinum group from high level wastes have been discussed by Kubota [45],... 

As observed from X-ray crystal data, which are available for a variety of complexes [4,5,6], oxotechnetium complexes show the following general feature the metal lies above the equatorial plane of the four basal ligand atoms and the oxo group is at the apex. Tc=0 bond lengths correlate with the displacement of the technetium atom from the basal plane towards the oxygen at the apical position. 

N-donor groups in the form of e-amino groups of lysine as nonspecific binding sites for technetium may play an undesired role in Tc-labelling of monoclonal antibodies for tumour imaging [51]. Nonspecifically bound Tc has a poor in vivo stability and appears to increase the undesired liver uptake and reduces tumour uptake. The metal oxidation state and coordination is not yet known. 

The inability of amines to deprotonate upon coordination, and thus to compensate the charge of the MOs+ core, can be overcome by combination with other types of donors-including N-donor groups-in the ligand, as has already been discussed above for oxocyclam. A prominent example is provided by technetium complexes of tetradentate amine oxime ligands. 

Technetium complexes with thioethers in the strict sense, i.e., those without other donor groups in the ligand molecule, comprise homoleptic thioether nitridotechnetium(V) complexes [111], cationic mixed thioether/thiolate complexes of Tc(III) [112], and a cationic Tc(I) complex [113]. However, these latter compounds do not properly fall within the scope of Tc(V) compounds and are excluded from review. 

Before complexes can be used in nuclear medicine, especially for brain function studies, in vivo reactivity has to be imposed upon them. Retention in the brain is necessary for perfusion imaging. This can be achieved by substituents on the ligand that bind them to binding sites in the brain. N-alkylation with methyl and other alkyl groups led to the class of complexes shown in Fig. 19. Upon complexation to technetium, the IV-alkyl substituent can assume a syn or anti configuration with respect to the oxo ligand, as proved by X-ray crystal... 

Tc(IV), the DTPA ligand is undoubtedly coordinated in a hexadentate fashion about the Tc through three nitrogens and three carboxylate oxygens, leaving two uncoordinated carboxylates. Deutsch and Packard [56], however, observed three uncoordinated carboxylates on the complex by titration with base, suggesting that the technetium may be in the 4- 5 oxidation state. If Tc(V) is correct, then a 6-coordinate complex would be present with a carboxylate in the position trans to the Tc = 0 group. 

The first two groups are distinguished by the fact that autoclave syntheses are used mainly by the Soviet (or Russian) cluster school [1,2,11,16-22], while the American and other schools do not use autoclave techniques [1,23-35]. This distinction is historical and applies not only to technetium compounds, but also to other cluster compounds of d-transition elements [36-41]. Cotton et al. [28] did use an autoclave method once, but it was not further developed [1]. 

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