Technetium polynuclear

Polynuclear Carbonyls. Several stmctures consist of dinuclear metal carbonyls as shown in stmctures (4)—(6). The metal atoms in Mn2(CO) Q, as also for technetium and rhenium, are held together by a metal—metal bond and the compound contains 10 terminal CO ligands, five coordinated to each atom. The CO ligands of Mn2(00) 0 adopt a staggered configuration as illustrated in stmcture... 

In compliance with the nuclearity principle, polynuclear clusters are subdivided into a number of other subgroups, e.g. hexanuclear, octanuclear, etc. The binuclear clusters of technetium may be classified according to the electronic structure of their Tc-Tc2 bonds. Then, the d4-d4 complexes with quadruple M-M bonds are the father of all binuclear complexes with Tc-Tc bonds. The addition or removal of electrons from Tc-Tc bonds [1,11] should result in a decrease in the formal multiplicity of M-M bonds. Thus, for instance, the formal multiplicity of Tc-Tc bonds of d3-d3 and d5-d5 binuclear complexes equals3 3, that of d4 d5 and d4-d3 complexes equals 3.5, etc. 

With autoclave syntheses a high yield of clusters is achieved, and it is possible for researchers to follow the reaction path in solution by gradually changing (from experiment to experiment) the working parameters of the synthesis (temperature, pressure, exposure at working temperatures, etc). All these advantages of the autoclave technique have resulted in an abundance of new forms of technetium clusters (particularly, polynuclear ones) because it has been possible to develop and improve the method of obtaining these compounds. 

The first communications about the synthesis [9,49] and structure [9,70,101] of polynuclear technetium halides appeared in 1982-83. These studies demonstrated the superior capacity of technetium to form polynuclear... 

Figure 5 shows a scheme for synthesizing all the bromides of polynuclear technetium clusters presently known. Table 2 summarizes details of their ap-... 

Table 2. Appearances and main properties of polynuclear bromide technetium clusters [11, 44, 53, 72, 76]...

The paper electrophoresis experiments carried out to study the mobility of polynuclear technetium clusters in aqueous solutions of HX of varying acidity, as a mobile phase, showed that these clusters were also characterized by reversible reactions such as (5) without leading to destruction of M-M bonds. On the other hand, an autoclave recrystallization of the polynuclear clusters at 200-220°C in an atmosphere of argon from concentrated solutions of HX led to a partial destruction of M-M bonds and the formation of binuclear complexes [Tc2X8]3 and [Tc,X6]2. This indirectly shows that reactions (6) and (7), leading to the destruction of M-M bonds, are likely in solutions of polynuclear clusters [15]. 

The cluster formation reactions considered in this review occur in technetium-concentrated solutions of hydrohalogenic acids. From comparison of the data reported in [9,11,44,49,50,53,68,72,73,75,76,118] it follows that the ions [Tc2X6]2b (X = Cl,Br) are immediate precursors to polynuclear technetium... 

Thus, it can be ascertained that polynuclear technetium clusters are formed from the binuclear ions [Tc2X8]4 and [Tc2X8]5 with the M-M bonds of formal order 5.0 and, probably, 4.5, respectively, i.e. under the conditions of reduction reactions at high concentrations of technetium ions the cycloaddition of multiple Tc Tc bonds takes place according to (26). 

Of special interest is the dehydration of polynuclear technetium bromide clusters, which contain hydroxonium cations with different numbers of hydration water molecules. Analysis of the results obtained leads us to conclude that at 140-200 °C dehydration occurs with a partial decomposition of the [H30(H20)3] + cations (30). 

Tc(IV) [or Tc(VII)] and Tc. An analogous tendency was observed for these clusters in aqueous solutions [11,80,87]. Polynuclear technetium clusters decompose with the formation of metallic technetium, i.e. the system of M-M bonds in this case appears to be more stable than the bonding of technetium with ligands. 

Electronic Structure of Polynuclear Technetium Clusters 5.3.1 Trigonal-Prismatic Clusters... 

The first and only nitrosyl hydride has been prepared recently by the reduction of the complex MnBr(NO)2(PPh3)2 with sodium boro-hydride (170). It may be noted that reduction of the nitrosylcarbonyl (ir-C5H5)Cr(CO)(NO)Br does not give a Cr—H complex. A possible polynuclear technetium hydride [Tc(CO)4]3H has been discussed (186). Reduction of the complex CH3SCH2CH2SCH3Mo(CO)3l2 affords an unstable hydride (215). 

Polynuclear complexes of low valent technetium have been extensively studied by Russian researchers. Halogeno complexes are selectively compiled in table 12.18.A. The compounds were prepared by reduction of HTCO4 in cone, hydrogen halide solutions with molecular hydrogen under a pressure of 3-5 MPa at 140-220 C [567j. 

Table I2.18.A. Selected polynuclear halogeno complexes of low valent technetium [567,568].

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