Transition metal complexes technetium

Technetium-99m coordination compounds are used very widely as noniavasive imaging tools (35) (see Imaging technology Radioactive tracers). Different coordination species concentrate ia different organs. Several of the [Tc O(chelate)2] types have been used. In fact, the large majority of nuclear medicine scans ia the United States are of technetium-99m complexes. Moreover, chiral transition-metal complexes have been used to probe nucleic acid stmcture (see Nucleic acids). For example, the two chiral isomers of tris(1,10-phenanthroline)mthenium (IT) [24162-09-2] (14) iateract differentiy with DNA. These compounds are enantioselective and provide an addition tool for DNA stmctural iaterpretation (36). 

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,... 

Vo er A, Kunkeley H (1990) Photochemistry of Transition Metal Complexes Induced by Outer-Sphere Charge Transfer Excitation. 158 1-30 Volkert WA, Jurisson S (1996) Technetium-99m Chelates as Radiopharmaceuticals. 176 123-148... 

The reliable prediction of redox potentials as a function of composition is useful in the synthetic design and application of technetium and other transition metal complexes. A parametric procedure for doing so on the basis of ligand additivity principles has been developed by Lever [28]. Lu etal. [29] used this scheme to correlateTc / ",Tc "/ , andTc hi potentials with the composition of octahedral technetium complexes containing halide, nitrogen, and phosphorus donor ligands. The results are illustrated in Fig. 2 [29], where the observed potentials are plotted according to... 

Vogler A, Kunkeley H (1990) Photochemistry of Transition Metal Complexes Induced by Outer-Sphere Charge Transfer Excitation. 158 1 - 30 Volkert WAJurisson S (1996) Technetium-99m Chelates as Radiopharmaceuticals. 176 123-148 Vondenhof M, see Mattay J (1991) 159 219-255 VoyerN (1997) The Development of Peptide Nanostructures. 184 1-38 Waldmann H, see Sauerbrei B (1997) 186 65 - 86 Walter C.see FessnerW-D (1997) 184 97-194 Wan P, see Krogh E (1990) 156 93-116... 

Transition metal complexes containing radioactive isotopes have found widespread use as radiopharmaceuticals in diagnostic nuclear medicine. The most useful transition metal nuclide is technetium-99m and there are many examples of the successful applications of complexes of Tc-99m in both structural and functional studies of patients. Limitations on the design of additional more useful and better technetium radiopharmaceuticals include a) a very incomplete knowledge of the chemistry and structure of technetium compounds and b) inadequate information on the factors which influence the biodistribution and interaction with biomolecules, such as receptors, of transition metal complexes. These factors are discussed in terms of the need for additional metalloradiophannaceuticals. 

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. 

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

Metal-metal (M-M) bonds, first noted in the early sixties, occur in several thousand transition-metal compounds [1]. Complex technetium compounds and compounds with M-M bonds (clusters) have been studied more extensively than many other classes of inorganic compounds. Increasing interest in technetium compounds is due to the practical uses of the "mTc isotope, which ranks first among radioactive isotopes used in nuclear medicine diagnostics [2-4]. On the other hand, technetium clusters are an interesting object for theoretical studies, because until recently, they were the only compounds in which the presence of these anomalous chemical bonds was thought possible. 

Over the last few years the number of complexes with heterocyclic carbene ligands has rapidly increased. Next to complexes of all transition metals [1, 2, 9, 73, 74], NHCs have been used as ligands for more exotic metals such as the /-block elements [92] and various technetium isotopes [93]. The search for NHC complexes is still driven by the need for new and selective catalysts [94]. 

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