Technetium redox reactions

Although these complicated features of the redox reactions of technetium are important topics, the present article is limited to the susbtitution kinetics of technetium complexes. Some redox kinetics of technetium complexes have been discussed by Koltunov and Gomonova [6], The redox potentials for analogous technetium and rhenium complexes, based on their differences, have been compiled by Deutsch et al. [7],... 

Such diagrams are extensively used in electrochemistry and can be borrowed from reference books such as the Atlas on Electrochemical Equilibria [2]. We will use these diagrams in redox reactions that form CBPCs, such as that of Fc203 (see Section 7.5, and also Chapter 12), and also for redox conditions used to stabilize contaminants such as technetium (see Chapter 17). In addition, these diagrams provide limits on use of the redox reactions because of limits on water stability. This point is discussed below. 

The decacarbonyls of manganese, technetium, and rhenium, of formula M2(CO)io, have terminal carbonyl groups and a metal-metal bond. The molecular symmetry is D d with the two M(CO)s fragments in a staggered conformation. The heterodinuclear decacarbonyl MnRe(CO)io is also known as obtained by the redox reaction of a rhenium pentacarbonyl halide with the pentacarbonylmanganate(-I) anion at room temperature (at 160 °C or higher, the heterodinuclear carbonyl tends to form the homodinuclear compounds with an equilibrium constant close to the statistical value ). The X-ray diffraction stndy of MnRe(CO)io has shown the Mn-Re distance of 2.909 A to be shorter than the sum of the covalent radii obtained from the homodinuclear compounds (Table 3). 

Introduction.—The organometallic chemistry of technetium and rhenium reported during 1974 has been surveyed. The ligand-induced redox reactions of rhenium halides have been reviewed, and the chemistry of cyanide complexes of Group Vila metals has received attention. The electrochemistry of technetium and rhenium has been the subject of two reviews, and a general monograph on the production, uses, and disposal of technetium has appeared. Recently published crystal structures of complexes of technetium and rhenium have been collated. ... 

Reported rhenium complexes possess one [70-74], two [75], and three [75] dithiocarboxylato ligands coordinated to the one rhenium atom to form a high coordination state, as in the case of the technetium complex [76]. One of these complexes 29 is synthesized from Re(VII)S4 and (PhCSS)2 (Fig. 10) [72]. An internal redox reaction occurs and Re( VII) is reduced to Re(III) in the synthetic process. Addition of sulfur-abstracting reagents, Ph3P or Et4NCN, produces the heptacoordinate complex 30 with the neutral capped octahedron structure or... 

PANI composites with the oxides of technetium and rhenium are scarcely reported. The formation of PANl/Ca-sihcate/rhenium oxide composite upon the uptake of pollutant ReO from aqueous solution by the nanostructured PANI/Ca-silicate composite via the redox reaction between PANI leucoemeraldine/emeraldine salt (reductant) and ReO (oxidant) was reported by Cairns [185] and Borrmann et al. [186]. 

Complex-formation and redox reactions have so far been the most frequently used for the determination of both metals and nonmetals (Table 3). A variety of transition metal ions have been thus determined including such common elements as iron, copper, and calcium and rare earths as well as technetium and europium. Common nonmetals such as nitrogen anions and phosphate, bromide, and sulfur anions have also been determined this way. However, the most important applications of noncatalytic reactions in inorganic analysis are the simultaneous determinations of metal ions. As can be seen in Table 4, a wide variety of binary mixtures and some ternary and even... 

Thiourea acts as a mild reductant (E° = 0.427 V) for transition metal complexes and is oxidized to formamidine disulfide. Thiourea is an excellent ligand for many metal ions, especially for those of the platinum metals.88 Therefore, in most of its redox reactions, thiourea acts simultaneously as a reductant and as a ligand. For example, OSO4 reacts with thiourea in sulfuric acid to form [Osfthiole] and formamidine disulfide.89 Addition of [NH4][Tc04] to a solution of thiourea in a mixture of ethanol and hydrochloric acid results in the formation of the homoleptic thiourea complex of technetium(in), [Tc(thio)6] ". An X-ray structure study of the isolated product, [Tc(thio)6]Cl3-4H20, has shown that the six thiourea ligands are S-bonded to the metal. 

Finally, while the new synthetic methods for technetium compounds in the oxidation states I, II and V will undoubtedly play an important role in the development of improved radioscintigraphic agents, there remains a need to understand the chemical mechanisms involved in these syntheses, particularly with regard to ligand substitution reactions and atom transfer redox processes. Mechanistic studies to complement the increasing body of structural knowledge is essential to the further development of technetium radiopharmaceuticals. 

The redox potentials given in Table 117 for technetium and rhenium are for the reactions ... 

Complex 4 and TpRe( = 0)Br2 (27) have been prepared by reaction of 3 with PPhs in the presence of Me3SiCl or Me3SiBr (Scheme 19). The reduction of 4 requires more forcing conditions relative to the technetium analog, consistent with the fact that reduction of rhenium complexes is typically less facile than corresponding technetium complexes. Replacement of chloride ligands with bromide does not significantly alter the redox properties of TpRe( = 0)X2 (X = Cl or Br). 

Technetium complexes with dtpa, dmsa, or mdp (methylene diphos-phonate) can be prepared by exchange reactions of the respective rhenium complexes with pertechnetate. Despite the complication that redox as well as substitution is involved here, rates correlate with metal-ligand bond strengths. A detailed kinetic study of these reactions would be welcome. Another type of ligand exchange reaction where kinetic studies are needed is the similar situation encountered in the preparation of technetium(III), (IV), or (V) complexes by reduction of pertechnetate with tin(II) complexes of the respective ligands. 

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