Transition metal cations water exchange

The structural and spectral complexity of clay minerals is sufficient to consider a single mineral as a multicomponent mixture in itself. Detectible by near infrared spectroscopy are adsorbed water and structural hydroxyls (25.) exchangeable and structural transition metal cations (2fL and this work), adsorbed species including atmospheric gases (22), organic materials (2) accessory minerals (2SL) and, possibly, trapped hole centers (0 -centers). Thus it is of interest to adapt NIRA to studies of mineral surface activity. We have done this by examination of a small set of highly homologous clays in which laboratory control of only one variable at a time could be accurately achieved and independently confirmed. 

Helm and Merbach have summarised efforts aimed at calculating exchange mechanisms for water exchange on first row, second and third row transition metal cations and on lanthanide ions. [26] The limitations within the calculations and where there is consistency with parameters obtained from ambient and high pressure kinetics results have been pointed out. An example of a particular theoretical investigation will be cited below. 

Selectivity of transition metal cations is not as straightforward as for the alkali and alkaline earth metals. Metal ions which have different valencies, or which have the same valency but are in different rows, can be separated by cation exchange. However, many transition metal cations have the same valency and therefore cannot be separated by cation exchange. One common solution to increase selectivity among transition metal cations is to form coordination complexes which form strong ionic species. In aqueous solutions some transition metals can hydrolize to form coordination complexes in which water is covalently bound. The water can be replaced by an ion or molecule which can donate one or more electron pairs to the metal. Such molecules are referred to as ligands and are classified by how many electron pairs they donate. Several anionic chelating molecules are shown in Table 5 which coordinate with many of the transition metal cations to form coordination complexes. 

Earlier reviews of the field were somewhat limited and focused on the introduction of transition metal cations [23] or alkaU metal and rare earth cations into zeohtes [24,25]. The present contribution intends to provide a more extended and almost complete overview of the state of the art and, also, to address more recent developments such as reductive solid-state ion exchange, related processes and questions concerning the role of water in contact-induced ion exchange, comparison of conventional and solid-state exchange, and incorporation of noble metals into narrow-pore zeolites. 

It is believed that clay minerals promote organic reactions via an acid catalysis [2a]. They are often activated by doping with transition metals to enrich the number of Lewis-acid sites by cationic exchange [4]. Alternative radical pathways have also been proposed [5] in agreement with the observation that clay-catalyzed Diels-Alder reactions are accelerated in the presence of radical sources [6], Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

Density functional theory has also been applied successfully to describe the solvent exchange mechanism for aquated Pd(II), Pt(II), and Zn(II) cations (1849 ). Our own work on aquated Zn(II) (19) was stimulated by our interest in the catalytic activity of such metal ions and by the absence of any solvent (water) exchange data for this cation. The optimized transition state structure clearly demonstrated the dissociative nature of the process in no way could a seventh water molecule be forced to enter the coordination sphere without the simultaneous dissociation of one of the six coordinated water molecules. More... 

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