Publisher Summary This chapter describes the coordination chemistry of technetium focusing on technetium(−I), technetium(0), technetium(I), technetium(II), technetium(III), technetium(IV), technetium(V), technetium(VI), and technetium(VII). Technetium(III) complexes with aminocarboxylato ligands have been observed but none are well characterized. 99m Tc–iminodiacetate complexes formed with 2,6-alkylphenyl [ArNHCOCH 2 N(CH 2 COO) 2 ] 2− ligands are used to image the hepatobiliary system. One of the approaches to cationic myocardial imaging agents has been the development of neutral seven-coordinate Tc(III) complexes based on 1,2-dioxime ligands (dioximeH 2 ) with one end capped by a boronic acid derivative. These complexes are generally referred to as boronic acid adducts of technetium dioximes (BATOs). The highest binary chloride of technetium is the dark red TcCl 4 formed as the major product of the chlorination of Tc metal. Crystallography reveals a polymeric chain structure of C1-bridged distorted octahedral TcCl 6 units. The structure and chemistry of the square-pyramidal five-coordinate and pseudo-octahedral six-coordinate [Tc V O] 3+ complexes are dominated by the strong tetragonal distortion induced by the multiply bonded oxo ligand.

The Coordination Chemistry of Technetium

X-ray absorption spectroscopy has been used to establish polymer formation of Tc(IV) in aqueous solutions of Na + SO 4 2- and Na + Cl - /SO 4 2-. As the molybdenum chemistry show similarities to that of technetium, we used MoO 2 as a reference to model our technetium species. Fitting of TcO 2 .xH 2 O with this model led to a good correlation with the literature data: (Tc-Tc = 2.53 A, Tc-O = 1.87-1.98 A). In aqueous solution, some polymers are formed regardless to the nature of the media composition: Tc-Tc = 2.50′ 0.02 A. The general structure is in agreement with a first coordination shell containing 6 O. The modelling shows that, in the first coordination shell, there is no chloride ligand. The observed geometries are close to those found for TcO 2 .xH 2 O, hence the unknown aqueous species must be considered as a precursor of the solid technetium dioxide. Combination of these results with XANES led to attribute Tc n I V O n ( 4 n - 2 p ) + (H 2 O) q with n > 2 to the species.

XAS study of technetium(IV) polymer formation in mixed sulphate/chloride media

The substitution of the Cl - ions of TcCl 6 2 - in various media (Cl - , SO 4 2 - , TFMS - ) at various pH or the electroreduction of TcO 4 - in the same media led to a common Tc(IV) species characterized by an UV-visible absorption band at 500nm. This band was present in the spectra of Tc(IV) in all the media investigated, except in chloride at low pH. The analysis of the spectroscopic data showed that it can be attributed to a Tc(IV) oxopolymeric species. The data fits better with a trimeric species Tc 3 O 4 4 + which reminds the Mo 3 O 4 .9H 2 O 4 + clusters known for Mo(IV). The stability of 3 × 10 - 5 M Tc 3 O 4 4 + solutions has been investigated as a function of pH with respect to the formation of Cl - complexes at low pH, and precipitation of TcO 2 at pH, higher than 3. The anions of the media interact with Tc 3 O 4 4 + clusters as shown by small changes in their UV-visible spectra. We observed that the affinity of Tc 3 O 4 4 + for these ions is in the order: SO 4 2 - > Cl > TFMS - . Neutral species Tc 3 O 4 (SO 4 ) 2 are observed in 0.1 M sulphate solutions.

Tc(IV) chemistry in mixed chloride/sulphate acidic media. Formation of polyoxopolymetallic species

The speciation of tetravalent technetium has been studied in HCl solutions ranging from 1.0 to 6.0 M. Tc(IV) exists in 6.0 M HCl as the well characterized TcCl62- chloride complex. In 1.0 M HCl, the chloride ligands of TcCl62- are slowly substituted by other ligands such as H2O, OH- or O2-. So TcClmOn(OH)p(H2O)q(4-m-2n-p)+ compounds may form with (m + n + p + q) = 6. Several speciation techniques such as UV-visible and Raman spectrometries, X-ray absorption spectroscopy and electrochemistry have enabled to get an insight into this problem. All the results have indicated two most likely species of Tc(IV) after aging for about ten days in 1.0 M HCl to be aquo-chlorocomplexes of technetium and not oxo-chorocomplexes: TcCl5 · H2O- and TcCl4 · 2 H2O.

The speciation of Tc(IV) in chloride solutions

The condensation mechanisms of tetravalent technetium in chloride media were studied in the pH range 0-1.5. A new dimer complex of Tc(IV) has thus been discovered, Tc 2 OCl 10 4- . Spectroscopic and kinetics studies showed that the formation of this compound resulted from the condensation of TcCl 5 (H 2 O) - . An EXAFS study indicates that the dimer displays a [Tc-O-Tc] 6+ structure. As the pH increases, UV-visible measurements showed a cyclization of [Tc-O-Tc] 6+ into [Tc(μ-O) 2 Tc] 4+ leading, in fine, to the precipitation of TcO 2 ·xH 2 O. The aquation constant (K aq ) of TcCl 6 2- into TcCl 5 (H 2 O) - and the dimerisation constant (log K dim ) of TcCl 5 (H 2 O) - into Tc 2 OCl 10 4- were determined to be 2.20±0.26 and 4.68±0.09, respectively.

Condensation mechanisms of tetravalent technetium in chloride media

The photochemical decomposition of (NH4)2TcCl6 in dilute HCl, HClO4 and H2SO4 solutions has been studied. Electrophoresis and spectrophotometry were used to identify TcCl
 6
 2−
 , TcCl5 (H2O)−, cationic and uncharged species and TcO
 4
 −
 . The yield of each species as a function of the time of uv irradiation was determined. The TcCl
 6
 2−
 , yield decreases to zero after 50 h of irradiation. The TcCl5(H2O)−, cationic and uncharged species and TcO
 4
 −
 are formed in different proportions in the three acids. After 120 h of irradiation of the HCl solution the principal Tc species was the cationic one (≈70%) followed by the uncharged species (≈24%). In HClO4 solution the cationic species (≈70%) was followed by TcO
 4
 −
 , but in H2SO4 solution the uncharged species amounts to ≈90%.

The aquation of hexachlorotechnetate(IV)

The products of the complexation of technetium by polyamino-carboxylic acids is still a controversal subject. The reduction of Tc99O4 - by various reducing agents (SnCl2, SO2, Sn) in the presence of diethylene-triamine-pentaacetic acid (DTPA) gives different products depending on the experimental conditions (1–3). The influence of pH, Tc concentration and Tc/ligand ratio on the reduction of aqueous pertechnetate by SnCl2 or SO2 in the presence of some aminocarboxylic acids, has recently been studied (2,3). It was shown that at concentrations of TcO4 - lower than 10-3 M both colourless and coloured products were formed. At higher TcO4 - concentration only coloured compounds were obtained (3).

M. Kosinski

Papers

The aquation of hexachlorotechnetate(IV)

New Aspects of Complexation of Technetium with DTPA Ligand