PHYSICOCHEMICAL APPROACH TO THE STUDIES OF METAL ALKOXIDES

The review provides a description of main events in the development of chemistry of metal alkoxides in Russian and summarizes the results ob tained by the authors. The studies of solubility and vapor pressure in the M(OR)(n)-ROH (R = Li, Mg, Ca, Sr, Ba, Tl) systems permitted the d etermination of the composition of the solvates formed and the optimiz ation of the isolation conditions for distinct derivatives. The electr ochemical synthetic approach to the alkoxide and 2-methoxyethoxide der ivatives of III-VIII Group elements has been elaborated. Improvement o f the MCln with NaOR metathesis conditions led to practically quantita tive yields of alkoxides. The examples of the structures of polynuclea r oxoalkoxides, earlier erroneously considered to be orthoderivatives, i.e. 'M(OR)(n)', are given. It has been stated that among the alkoxod erivatives of Zr and I-If these are only M(OC2H4OMe)(4) and M(OR)(4) . ROH (R = Pr', Bu') that exist as individual compounds, while the samp les of those with different R contain oxocomplexes of M3O(OR)(10) and M4O(OR)(14) composition. The questions connected with the origin of ox ogroups in the molecules of alkoxides and their influence on the prope rties of the samples are discussed. The irreproducibility of the physi cochemical constants for the samples (physical state, melting points, solubility in alcohols, intensity of coloration) is caused by differen t 'chemical' (synthetic procedures, isolation conditions) and thermal prehistory and storage times; it originates from the difference in the ir molecular composition (different fractions of different types of ol igomeric and polymeric [M(OR)(n)](m) and MxOy(OR)(z) aggregates). Form ation of bimetallic alkoxides is studied using the plotting of 20 degr ees C solubility isotherms in M(OR)(n)- M'(OR)(m)-Solvent systems. Dif ferent kinds of systems are considered and the composition and structu re of bimetallic complexes and oxocomplexes, originating from their de composition, are described. Decomposition of alkoxomolybdates and -tun gstates in solutions may result in crystallization of complex oxides a s the final products. The example of MTiO3 (M = Mg, Ba) oxide preparat ion is discussed as most illustratively demonstrating the need in stud ies of metal alkoxides interaction in solution for the optimization of synthesis of oxides by the sol-gel technique. (C) 1998 Elsevier Scien ce Ltd. All rights reserved.