Metals coordination with water

The lanthanide ions are so large that they show coordination numbers of 10, 11 and even 12. Hoard and his students have shown that when ethylenediaminetetraacetate (edta) combines with the larger lanthanide ions, the sexidentate edta covers only a portion of the metal ion, the remainder being coordinated with water molecules.18... 

Hydrolysis of metal alkoxides is the basis for the sol-gel method of preparation of oxide materials therefore, reactions of metal alkoxides with water in various solvents, and primarily in alcohols, may be considered as their most important chemical properties. For many years the sol-gel method was mosdy associated with hydrolysis of Si(OR)4, discussed in numerous original papers and reviews [242, 1793,243]. Hydrolysis of M(OR) , in contrast to hydrolysis of Si(OR)4, is an extremely quick process therefore, the main concepts well developed for Si(OR)4 cannot be applied to hydrolysis of alcoholic derivatives of metals. Moreover, it proved impossible to apply classical kinetic approaches successfully used for the hydrolysis of Si(OR)4 to the study of the hydrolysis of metal alkoxides. A higher coordination number of metals in their alcoholic derivatives in comparison with Si(OR)4 leads to the high tendency to oligomerization of metal alkoxides in their solutions prior to hydrolysis step as well as to the continuation of this process of oligomerization and polymerization after first steps of substitution of alkoxide groups by hydroxides in the course of their reactions with water molecules. This results in extremely complicated oligomeric and polymeric structures of the metal alkoxides hydrolysis products. 

Although it is obvious that anions do react in the same way as metal cations with water, the anions are treated merely as monoprotonated species that may either compete with the water (HX) or with the hydroxyl ions (monodentate anion X ) in the coordination sphere. The electronegativity can be... 

Following the well-known Pinner synthesis of imidates by the reaction of nitriles with alcohols, there has been considerable interest in recent years in treating a nitrile bond within the coordination sphere of metal ions with water or alcohols. It has been suggested that the initial step consists of nucleophilic attack of an external or coordinated hydroxide or alkoxide anion on the nitrile carbon atom. A number of imidate complexes have been isolated from the reactions of 2-cyanopyridine with metal [Cu", Ni , Co" and Fe"] salts in alcohols, of frans-PtMeClL with pentafluorobenzonitrile and silver hexafluorophosphate in alcohols," and of o-cyanobenzylplati-num complexes. "... 

Some species have a large number of exchangeable hydrogens. Thus, metal ions with many coordinated water molecules will become completely deuterated in D20. For example, Co(H20) + becomes Co(D20) +. If there were a 2 percent secondary kie per bond, it would be amplified to (1.02)12 or 1.27. It would not be a simple matter to demonstrate the operation of a primary kie in such a system because of this, given the p/f difference between the two metal ions and the general effects of the solvent change. 

Ce4+ is a versatile one-electron oxidizing agent (E° = - 1.71 eV in HC10466 capable of oxidizing sulfoxides. Rao and coworkers66 have described the oxidation of dimethyl sulfoxide to dimethyl sulfone by Ce4+ cation in perchloric acid and proposed a SET mechanism. In the first step DMSO rapidly replaces a molecule of water in the coordination sphere of the metal (Ce v has a coordination number of 8). An intramolecular electron transfer leads to the production of a cation which is subsequently converted into sulfone by reaction with water. The formation of radicals was confirmed by polymerization of acrylonitrile added to the medium. We have written a plausible mechanism for the process (Scheme 8), but there is no compelling experimental data concerning the inner versus outer sphere character of the reaction between HzO and the radical cation of DMSO. 

The majority of ligands are either neutral or anionic. Those which coordinate to a metal ion through a single atom are described as monodentate or unidentate. Examples of such ligands which we have encountered thus far include water, ammonia and chloride. A more extensive listing of common ligands is found in Table 1-3. We stress at this point that there is no difference in kind between the interactions of a metal centre with either neutral or anionic ligands. 

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