Polynuclear clusters

This example shows that dipolar interactions can produce unexpected effects in systems containing polynuclear clusters, so that their complete quantitative description requires a model in which the dipolar interactions between all the paramagnetic sites of the system are explicitly taken into account. Local spin models of this kind can provide a description of the relative arrangement of the interacting centers at atomic resolution and have been worked out for systems containing [2Fe-2S] and [4Fe-4S] clusters (112, 192). In the latter case, an additional complication arises due to the delocalized character of the [Fe(III), Fe(II)] mixed-valence pair, so that the magnetic moments carried by the two sites A and B of Fig. 8B must be written... 

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. 

At present this is the most widespread system of classification of bi- and polynuclear clusters. 

It is noteworthy that all these reactions are reversible, but under reducing conditions the equilibrium shifts to the right-hand side, and under oxidizing conditions it shifts to the left-hand side. For all resulting binuclear ions at rather low concentration in solutions, reactions take place, accompanied by the rupture of the M-M bond and the formation of unstable mononuclear complexes, which are readily oxidized even by the solvent. At high concentrations of binuclear ions cycloaddition reactions of multiple M-M bonds (these reactions are considered in section 3.5) occur to form stable, under the given conditions, polynuclear clusters. 

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]. 

For polynuclear clusters similar reactions were not obtained These reactions proceed in the forward direction only at high temperatures ( 120 °C) and in the presence of an excess of the bidentate complex-forming agent, whereas the reverse reactions (20-22) also take place at room temperature in the presence of excess HC1 [11,46,49],... 

These reactions are irreversible organic acid ions apparently behave in the reactions as oxidants, being converted into the corresponding alcohols and their esters [11,48], Reactions (23) and (24) take place only at high temperatures (> 150°Q and in the presence of a large excess of organic acid [11,48,58], These reactions are also possible for polynuclear clusters, because redox reactions occur readily and reversibly for them [50],... 

These processes include the thermal decomposition of all polynuclear clusters and also binuclear clusters with organic cations and ligands (Table 3). As a rule,... 

As already observed for some isotropic polynuclear clusters [30 - 32], slow relaxation of the magnetization in an external magnetic field can occur because of the inefficient transfer of energy to the environment, for example, the helium bath, and consequent reabsorption of the emitted phonon by the spin system. The phenomenon, also known as phonon bottleneck (PB), was first introduced by Van Vleck [33]. It is characteristic of low temperatures, where relaxation is dominated by the direct process between closely spaced levels, and results from the low density of phonons with such a long wavelength to match the small energy separation... 

An important and convenient method274 to synthesize triphenylgermanium halides, useful for the preparation of polynuclear clusters, is the reaction of PhMgBr with GeCLj in toluene ... 

Abstract This review deals with spin crossover effects in small polynuclear clusters, particularly dinuclear species, and in extended network molecular materials, some of which have interpenetrated network structures. Fe(II)Fe(II) species are the main focus but Co(II)Co(II) compounds are included. The sections on dinuclear compounds include short background reviews on (i) synergism of SCO and spin-spin magnetic exchange (ii) cooperativity (memory effects) in polynuclear compounds, and (iii) the design of dinu-... 

In this context, much effort has also been invested in controlling the nuclearity of the catalyst ensemble through the selection of its precursor. One area in which considerable progress has been made involves the adsorption of polynuclear clusters onto supports [33]. Examples involving the immobilization of small, preformed polynuclear clusters on supports are the reactions of carbonyl clusters of the late metals [16, 34], the binding of polyoxometalates (POMs) and their neutral alkoxy analogues [35] and heteropolyacids such as the Keggin cluster [36, 37]. 

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