Technetium lower oxidation state complexes

This section is actually devoted to the description of rhenium alkoxides, as the technetium ones are rarely studied. The latter are reviewed in a short appendix at the end. The chemistry ofrhenium in lower oxidation states is much alike that of ruthenium and therefore even for rhenium, the lower oxidation state complexes with jt-acceptor ligands are described in this chapter. 

The O-donor complexes of Tc(V) exhibit moderate and differential stability in aqueous solution. In the presence of reducing agents, such as stannous chloride, they are reduced to mainly undefined products of Tc in a lower oxidation state. However, at the low technetium concentration of "mTc that is used in nuclear medicine, the rate of the reduction process is very low. This makes it possible to prepare Tc(V) radiopharmaceuticals with O-donor ligands by the usual procedure, in which an excess of reducing agent over technetium is unavoidably used. The Tc(V) complexes also tend either to be easily oxidized or to disproportionate [23],... 

The extraction of technetiiun by different organic solvents is used in numerous separation and concentration procedures. Technetium is extracted as pertechnetate or, in lower oxidation states, as a complex compound. TcO can be extracted by the following main types of reactions ... 

Complexes of technetium in oxidation states ranging from (-1) to (VII) have been prepared chemically and characterized. However, historically only the higher oxidation states (IV), (V) and (VII) have been of major importance in radiopharmaceutical formulations. More recently there has been increased interest in lower oxidation state technetium complexes for medical applications, and the use of ir-acceptor ligands has allowed the preparation of Tc1 complexes which are stable in vivo. The coordination chemistry of technetium has been described in Chapter 42 and recent reviews have been provided by Davison21 and by Schwochau.22 Reviews which relate to medical applications of technetium are given by Jones and Davison,549 Deutsch et al.,20 Deutsch and Barnett,550 Siedel551 and Clarke and Fackler.552 The in vivo chemistry of "mTc chelates has been described by Eckelman and Volkert,553 while the structures of technetium complexes, determined by X-ray diffraction techniques, have been reviewed by Bandoli et ai554... 

The chemistry of technetium is quite complex especially because of its numerous oxidation states (4-7 to -1). In the higher oxidations states (Tc(IV) to Tc(VII)), the metal behaves as a hard Lewis acid and its chemistry is dominated by multiple-bonded oxo ligands. The lower oxidation states ions can be classified as soft metals and will form stable compounds with hgands containing the following donor atoms P, As, S, Se, X (except F ), N, and itt-acceptors such as CO, NO, and isonitriles. 

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