Technetium IV

The cations have a marked effect upon the rates of the reactions, and rates were measured in Na+ or K+ solutions. The observed rate constant (k) is given by equation (15)  

Manganese(iii).— The rates of oxidation of thiourea derivatives and of benzene-1,2-dithiol by the [Mn(OH)] + ion have been shown to be controlled by the rate of substitution of the substrate at the ion,  

K2[TcFf,] was prepared by fusion of K2[TcBr(,l wath excess KHF2. Recrystallization of potassium hcxafluorotcchnetatc(lV) from water gave pale pink platelets. The resistance of [TcF(i - against hydrolysis is remarkable, only cone, alkaline solution decomposes the compound by precipitation of Tc02-hydratc. The solubility of K2[ rcF6l 

Some crystallographic and magnetic data of Tc(IV) hexahalogeno complex salts are collected in Table 12.11.A. 

Complex. salt Structure Lattice constants IA Magnetic Moment B.M. Weiss constant ftl] Ref. 

Complex salt Structure Lattice constants lA] Magnetic Moment B..M.l Weiss constant 6 ] Ref. 

5 M HBr is 8.96-10 mole/1 [327]. In aqueous solution [TcBr ]- is readily decomposed by precipitation of Tc02-hydratc. The stepwise formation constant k(, of [TcBrs]- in 3 M HCIO4 is 3.8 10 mole 1 at 15 °C, which is almost 50 times smaller than that of [ReBrs], measured potentiometrically at the same conditions [331]. Ihe force constant of the Tc-Br bond in K2[TcBr5] was determined in the valence force field to be/r=1.39 mdync A [321]. The cubic ligand field parameter of [TcBiy,] was derived as d=18.7 10 cm [332] and d=21.6 HP cm [342]. 

Complexes with Oxygen Ligands and Oxo-Bridged Complexes 

The unexpectedly high solubility of K2[TcBr6] (in contrast to that of the fluoro and chloro complexes) in methanol appears to be due to partial substitution of Br by MeO . By the use of KOR, salts such as K2[Tc(OMe)6] and K2[Tc(0CH2CH20)3] may be isolated. These complexes show a f(TcO) IR absorption at 450-460 cm-1 (356). A crystal structure of 28, with the zwitterionic tripod ligand Me3N+C(CH20 )3, shows octahedral geometry with Tc-0 distances of 1.987(4)-2.005(4) A. Complex 28 is water soluble and stable at pH 4 for over 24 hr (357). 

Reduction of Tc04 in the presence of aminocarboxylic and carboxylic acids or substitution of [TcX6]2 in aqueous solution leads to the formation of bis(/x-oxo) Tc(IV7IV) or Tc(III/IV) dimers. Structural and IR data are listed in Table I and the structure of the oxalato complex is shown in Fig. 10. The four-membered Tc(/x-0)2Tc ring is near planar in all cases and the short Tc-Tc distances are consistent with a multiple 

Complexes with Schiff Base and Other Nitrogen Ligands 

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The main difference between the structures of compound (IV) and compounds (I), (II), and (III) is that in compound (IV) half of the bridging and terminal bromine atoms, and all the axial bromine atoms, are substituted by iodine atoms, with partial substitution being statistical [75]. Thus, according to the X-ray diffraction data, the effective Tc-Tc distances are intermediate between analogous distances in bromide and iodide complexes of technetium(IV) (Table 1) [104,105]. We observed that the substitution of I for Br has almost no effect on the Tc-Tc distances. 

Thus partial evaporation of HC1 during the synthesis of [Tc2C18]3 by the method of Eakins et al. [24] and by the autoclave method [22,42,43] creates favorable conditions (from the standpoint of the acidity of the solution and the concentration of technetium ions in it) for the formation of the octachloro-ditechnetates (+2.5), since, on the one hand, the rate of reduction of the technetium(IV) ions increases owing to their hydrolysis [42] and, on the other hand, the stability of the hydrolyzed cluster ions formed increases in relation to the reactions involving disproportionation and oxidation by atmospheric oxygen [9,52,80,87]. We may note that under the conditions of more pronounced hydrolysis, the rate of reduction of technetium(IV) increases so much that the formation of metallic technetium becomes possible. 

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 ... 

Ballestra et al. [32] described a radiochemical measurement for determination of "technetium in rain, river, and seawater, which involved reduction to technetium (IV), followed by iron hydroxide precipitation and oxidation to the heptavalent state. Technetium (VII) was extracted with xylene and electrode-posited in sodium hydroxide solution. The radiochemical yield was determined by gamma counting on an anticoincidence shield GM-gas flow counter. The radiochemical yield of 50 to 150 litre water samples was 20-60%. 

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%. 

The potential of the Re -Re couple in LiCl-KCl eutectic at 500 °C is — 0.358 V vs. the standard Pt electrode). 2 2 oxidizes [TcClg] directly to TCO4, but CI2 first produces technetium( v) which is then oxidized to pertechnetate. No oxidation of technetium(iv) by atmospheric oxygen in the presence of sunlight could be achieved. 

Partition behaviour of americium(III) chelates with cupferron and other bidentate reagents was studied spectrophotometrically between a number of inert solvents and dilute HC104 solutions.98 Of special interest may be the data on their extractability and colours of chloroform extracts, collected in a tabular form for cupferronate derivatives of 58 metals. The pH ranges for the formation of cupferronates of 39 metal ions have been shown graphically in this publication.99 Solvent extraction and polarographic techniques were employed to study the possible adducts between technetium and cupferron.100 Evidence indicates a Tcm cupferronate and possibly a pertechnitate adduct, but no indication of a technetium(IV) complex was obtained. 

Table VH shows that hydrazine is the only important variable for technetium sorption on each of the geologic solids. Hydrazine causes technetium to be removed from solution either by sorption or by precipitation of the reduced technetium species. Hydrazine is a powerful reducing agent and should reduce TcO/ to technetium(IV) according to standard half-cell potentials. No Tc02 was observed however, since the technetium passed through...

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