Technetium O

Only a few complexes of technetium in the zero oxidation state are known, and are exclusively formal derivatives of ditechnelium decacarbonyl. Heteronuclear decacar-bonyls have been synthesized in which one T c nucleus is substituted by Co, Mn. or Re. The carbonyl groups in Tc2COio]° were partially or completely replaced by PF3 or one CO group could be substituted by a carbene ligand. Also, the butadiene- 

The heteronuclear decacarbonyls ITcMn(CO)io] and [TcRe(CO)io] were synthesized by reaction of a pcntacarbonyl anion with the respective pentacarbonyl bromide  

A solution of [Mii2(CO)io or (Tc2(CO)io]° in THF was reduced with excess sodium amalgam to produce the pcntacarbonyl anions to which the corresponding pcntacarbonyl bromide in THF was added slowly. [TcMn(CO)io]° and [TcRe(CO)ui]° were readily separated on nonpolar columns by gas chromatography from by-products and w ere identified by IR spectroscopy and mass spectrometry. The carbonyl stretching frequencies for the metal and mixed metal decacarbonyls of Mn, Tc, and Re are compiled in Table 12.29. A. 

The technetium-cobalt carbonyl TcCo(CO)9]° was prepared in a similar procedure by reacting [Tc(CO)5Br with the tctracarbonyl cobaltate anion [Co(CO)4] in THF  

Substitution reactions of Tc2(CO)in]° with PF3 were observed under ihermal conditions at 1 lO C and under photolytic conditions at room temperature using a 450 W medium-pressure mercury lamp. Thermal reactions produced compounds with up to six of the carbonyls replaced by trifluorophosphinc, photolytic reactions yielded prod- 

Substitution of the CO ligands in [Tc2(CO)10] by the strong 7r-acceptor PF3 is achieved either thermally or photolytically. In one study up to eight CO ligands were replaced to give at least 24 [Tc2(CO)10 (PF3)J 

Complexes containing cyclopentadienyl and related ligands are considered in Section B. 

Complexes containing from one to six carbonyl groups are known and all obey the 18-electron rule. The colorless salt [Tc(CO)6]A1C14 is formed by the reaction of [Tc(CO)5C1] with A1C13 under 300 atm CO pressure and is soluble in THF, acetone, and methanol and stable in aqueous solution (65). The carbonyl halides [Tc(CO)5X] (X = Cl, Br, I) may be prepared by the reaction of the halogen with [Tc2(CO)10]. Reaction with chlorine and bromine occurs readily at room temperature but reaction with iodine is extremely slow. The iodide has been prepared by the high-pressure carbonylation of [Tc(CO)4I]2 (45). An alternative 

Complexes based on the [Tc(CO)4] core are [Tc(CO)4(S2CNR2)] (R = Me, Et), the cationic [Tc(CO)4(PPh3)2]AlCl4 (65), and [Tc(CO)4(acac)] (68). The dithiocarbamato complexes are formed by the reaction of Na(S2CNR2) with [Tc(CO)5C1] in acetone or THF. Grinding of [Tc(CO)5Cl] with K(jS-diketonate) under a layer of CC14 yields the unstable tetracarbonyl /3-diketonates (68). 

Reaction of [HTc(N2)(dppe)2] with CO in benzene or with methanol in the presence of pyridine gives [HTc(CO)(dppe)2], In the latter reaction methanol serves as the source of CO. On reflux in MeCN, [HTc(CO)-(dppe)2] is converted to [Tc(CO)(MeCN)(dppe)2]PF6 (85). 

A notable feature of this oxidation state is that a considerable number of Tc and Tc complexes can be prepared in high yields in aqueous media (36). As a consequence the coordination chemistry of Tc(I) has been intensively investigated in the search for Tc cationic myocardial imaging agents. Tc(I) complexes have the low-spin d configuration and are diamagnetic. The 18-electron rule is generally applicable and nicely explains the stability and the prevalence of six-coordinate complexes. 

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Gable, Kevin P.. Rhenium and Technetium O.vo Comple.ves in the Study ofOryanic O vidation Met hanisms. 

Until 1960, technetium was available only in small amounts and the price was as high as 2800/g. It is now commercially available to holders of O.R.N.L. permits at a price of 60/g. 

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). 

The isolation and identification of 4 radioactive elements in minute amounts took place at the turn of the century, and in each case the insight provided by the periodic classification into the predicted chemical properties of these elements proved invaluable. Marie Curie identified polonium in 1898 and, later in the same year working with Pierre Curie, isolated radium. Actinium followed in 1899 (A. Debierne) and the heaviest noble gas, radon, in 1900 (F. E. Dorn). Details will be found in later chapters which also recount the discoveries made in the present century of protactinium (O. Hahn and Lise Meitner, 1917), hafnium (D. Coster and G. von Hevesey, 1923), rhenium (W. Noddack, Ida Tacke and O. Berg, 1925), technetium (C. Perrier and E. Segre, 1937), francium (Marguerite Percy, 1939) and promethium (J. A. Marinsky, L. E. Glendenin and C. D. Coryell, 1945). 

In the solid state all three elements have typically metallic structures. Technetium and Re are isostructural with hep lattices, but there are 4 allotropes of Mn of which the o-fomi is the one stable at room temperature. This has a bcc structure in which, for reasons which are not clear, there are 4 distinct types of Mn atom. It is hard and brittle, and noticeably less refractory than its predecessors in the first transition series. 

The +4 Oxidation state ls the only uae in which all three elements form stable oxides, but only m the c.ase of technetium is this the most stable oxide. TcOz is the hnal product wi n any Tc/O... 

Erdal, B.R. Aguilar, R.D. Bayhurst, B.P. Daniels, W.R. Duffy, C.J. Lawrence, F.O. Maestas, S. Oliver, P.Q. Wolfsberg, K. "Sorption-Desorption Studies on Granite. I. Initial Studies of Strontium, Technetium, Cesium, Barium, Cerium, Europium, Uranium, Plutonium, and Americium", in "Proceedings of the Task 4 Waste Isolation Safety Assessment Program Second Contractor Information Meeting", Vol. II, Report PNL-SA-7352, Battelle Pacific Northwest Laboratory, 1978, pp. 7-67. 

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],... 

Schiff bases provide useful mixed donor sets. The carbonyl function of the most frequently used ligands is derived from either 1,3-dicarbonyl compounds or salicylaldehyde. Favourable combinations involve O-, N- and S-donor atoms. A range of technetium and rhenium complexes exist with bi-, tri-, tetra- and pentadentate ligands. The geometry of these complexes depends on the number and type of coordinating atoms as well as on the chain length between the donor atoms in the SchifF-base ligands. 

Whatever the advantages of the SCF calculations with the Cl s, they do not always give good results, as is the case for M2 molecules with quintuple and sextuple M-M bonds [135,156], Nevertheless, some deviations from the simple MO scheme can be explained using simpler qualitative arguments. For example, the increased ability of technetium to form d4-d5 complexes and their greater stability in comparison to that of the d4-d4 complexes are explained on the basis of a model of the electrostatic repulsion of M atoms with like charges in a binuclear cluster [10,90,150] or on the basis of different diffusivities of cr, o, n, 5 and S metallic MO s of the clusters [63,141,157]. 

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. 

Kodina G, Tulskaya T, Gureev E, Brodskaya G, Gapurova O, Drosdovsky B (1990) in Nicolini M, Bandoli G, Mazzi U (eds) Technetium and Rhenium in Chemistry and Nuclear Medicine 3. 

Technetium (Tc, [Kr]4 /65.vl), name and symbol after the Greek Tsxrmos (tech-nikos, artificial). Detected in Italy (1937) by Carlo Perrier and Emilio Segre in a sample of Mo which had been irradiated with deuterons at the E.O. Lawrence cyclotron in California. It was the first artificially produced element. 

The silane o-complexes 11 have been intensively studied as discussed above and in Refs. 2, 4, and 12-14. No examples of such complexes are known for technetium, whereas the related rhenium complex [Re(H)(SiR3)(CO)2Cp] has been concluded to be classical on the grounds of a long-estimated Si-H distance of 2.2 A. Schubert favors a classical description of the latter compound, whereas Kubas noted that this distance may correspond to a stretched a-complex on the verge of oxidative... 

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