Technetium complexes determination

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 most fundamental approach to assessing lability of complexes is by determination of the rate of isotopic exchange reactions. In the technetium-complex systems, no study of the exchange reaction on the central metal ion has been reported, but several reports have been published on isotopic exchange by ligand substitution. 

The importance of technetium-99m radiopharmaceuticals stimulated numerous studies related to the basic chemistry of long-lived technetium Tc. A great number of technetium complexes reviewed in the preceding sections has been synthesized and characterized. Exact technetium analyses in these compounds have been needed to confirm their composition. Besides the traditional determinations of Tc by conductometric titrations of pertechnetate or by liquid scintillation counting, neutron activation of Tc has also been... 

Using the cascade of 0.59-0.54. VIeV emitted by " Tc, Tc was determined in samples of 1-2 mg of technetium complex compounds by neutron activation analysis. Moderate sample activities were obtained after an irradiation time of 20 s at a neutron flux of about 10 cm "-s Good agreement between expected and found Tc contents was observed. The uncertainty of the mean value could be restrieted to less than 1 % ]26]. 

Most technetium complexes are redox-active and, because of their low-spin electronic configurations, electron transfer to and from technetium complexes is generally facile [8,9]. Technetium(II) is a particularly accessible oxidation state since it can be readily generated by the oxidation of stable Tc(I) complexes or by the reduction of stable Tc(III) complexes. In vivo reduction of Tc(IU) cations to their neutral Tc(II) counterparts has proven to be an important factor in determining the biological distributions of these cations [10,11]. 

Technetium compounds with amine/thioether coordination are the cationic trans-dioxotechnetium(V) complexes [Tc02(N2S2)]+. The complex in which N2S2 is l,4-dithia-8,ll-diazacyclotetradecane was prepared via an exchange reaction of NBu4[TcOBr4] with the ligand and fully characterized by X-ray crystal structure determination [109,110]. The coordination around technetium... 

The rates of ligand exchange of 0.01 M hexahalo-technetium(IV) and hexa-halo-rhenium(IV) complexes were measured in the 8 M solutions of their corresponding acid at 60 °C. The overall exchange rates R were determined as follows ... 

The strong absorptions of the complex technetium (IV) hexahalides (Fig. 10) can also be utilized for spectrophotometric determinations. A sensitive method has been developed using hexachlorotechnetate (IV) When pertechnetate is heated for 50- 0 min in cone, hydrochloric acid, it is reduced to the complex [TcClgp . The absorption curve of [TcClgf in cone. HCl has a maximum at 338 nm where technetium can be determined in the presence of microgram amounts of rhenium or molybdemun. The molar extinction coefficient is said to be 32.000 (after Jorgensen and Schwochau it amounts to 10.600). About 0.1 fig Tc/ml can be determined. Rhenium present in quantities up to 30 ng/ml has almost no influence on the determination of technetium. The error in the determination of the latter in the presence of molybdenum at a weight ratio of 1 1 is 1-2%. 

Foster et al. have developed a method for determining technetium in dissolved nuclear fuel solutions. Tetrapropylammonium pertechnetate is doubly extracted from a basic medium into chloroform and the colored technetium (V) thiocyanate complex is formed in the chloroform phase by the addition of sulfuric acid, potassium thiocyanate and tetrapropylammonium hydroxide. The colored complex absorbs at 513 nm, has a molar extinction coefficient of 46,000 and is stable for several hours. Of more than 50 metals studied, none impairs measurements at ratios less than 100 to 1 mol with respect to technetium. Most anions do not disturb the determination of technetiiun. The standard deviation for a single determination is 0.09 fig over the range of 1 to 20 fig of technetium. 

Miller and Zittef have used 1,5-diphenylcarbazide (0.25% solution in acetone) for the spectrophotometric determination of technetiiun. 1 to 15 /ig of technetium in 10 ml solution can be ascertained by measuring the extinction at 520 nm of the Tc (IV) complex in 1.5 M sulfuric acid. The development of the most intense color takes about 35 min the reduction of pertechnetate to Tc (IV) is effected by the reagent itself before complexation occurs. The molar extinction coefficient of the complex at 520 nm is 48,600. The relative standard deviation is 2%. Fe ", Ce ", and CrOj" clearly disturb measurements, VO , MoOj ,... 

Pertechnetate forms a blue complex and perrhenate a brownish-yellow complex with K4[Fe(CN) ] in presence of bismuth amalgam. This permits the spectrophotometric determination of both elements in the same solution . The adsorption maxima of the technetium and rhenium complexes are at 680 and 420 nm, respectively. The molar extinction coefficients are 10,800 for technetium and 4,000 for rhenium. Metals forming color or precipitates with K4[Fe(CN) ] must first be removed. 

Technetium(v) and Rhenium(v) Complexes.—Re2Tc5 has been obtained by ampoule synthesis from the elements at 800 °C, and its thermodynamic constants and X-ray diffraction characteristics determined/ Re2S3Cl4 has been identified in the Re—S—Cl system. Chloride complexes of technetium(v), K2[TcOCl5] and K2[TcO(OH)Cl4], have been obtained from a solution ofKTc04in HC1. "... 

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