Oxidized metals, high pressure effects

The discussion above has been directed principally to thermally induced spin transitions, but other physical perturbations can either initiate or modify a spin transition. The effect of a change in the external pressure has been widely studied and is treated in detail in Chap. 22. The normal effect of an increase in pressure is to stabilise the low spin state, i.e. to increase the transition temperature. This can be understood in terms of the volume reduction which accompanies the high spin—dow spin change, arising primarily from the shorter metal-donor atom distances in the low spin form. An increase in pressure effectively increases the separation between the zero point energies of the low spin and high spin states by the work term PAV. The application of pressure can in fact induce a transition in a HS system for which a thermal transition does not occur. This applies in complex systems, e.g. in [Fe (phen)2Cl2] [158] and also in the simple binary compounds iron(II) oxide [159] and iron(II) sulfide [160]. Transitions such as those in these simple binary systems can be expected in minerals of iron and other first transition series metals in the deep mantle and core of the earth. 

In these polymer-metal complexes of the Werner type, however, organometallic compounds are formed as reaction intermediates and/or activated complexes. As a result, the properties of polymer-metal catalysts in reductive reactions are different from those of polymer-metal catalysts in oxidative reactions. In the former, the catalytic reactions are very sensitive to moisture and air, and the complex catalysts often decompose in the presence of water and oxygen. Thus, reductive catalytic reactions are carried out under artificial conditions such as organic solvent, high pressure, and high temperature. Oxidative catalytic reactions, on the other hand, proceed under mild conditions aqueous solution, oxygen atmosphere, and room temperature. Therefore, it is to be expected that the catalytic effects of a polymer ligand will differ from the latter to the former. 

The investigations of platinum pyrochlores have demonstrated the effectiveness of high pressure techniques in the synthesis of anhydrous oxides when one or both reactants have limited thermal stability. The bulk of the work reported here represents a continuation of an exploration of metal oxide-platinum oxide systems at high pressure. 

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