SPECIFIC FEATURES OF PHYSICO-CHEMICAL BEHAVIOR COORDINATION POLYMERISM

SPECIFIC FEATURES OF PHYSICO-CHEMICAL BEHAVIOR COORDINATION POLYMERISM  

The structural types described in Chapt. 4 correspond to the crystalline alkoxides. At the same time, M(OR) are much more prone to give amorphous samples due, supposedly, to the nonhomogeneity of their molecular composition. Crystallization occurs only in the cases when the samples contain only one type of aggregate or a regular polymeric structure. 

The aging processes leading to the changes of the properties of samples overtime (such as solidification of liquids, decrease in solubility, increase in evaporation temperature, changes in the composition of the fragments in the mass-spectra, crystallization of amorphous solids) have become wellknown. These processes, very characteristic also for the hydroxides, are caused by aggregation or polymerization and also by condensation of oligomeric molecules. 

During the last 30 years the literature has included many reports on the variation of the properties (such as m., solubility, vapor pressure, IR, and mass-spectra) of samples of different alkoxides in time. Such examples were provided by the samples of Al(OEt)3and Al(OPr% [1640, 1642], Ga(Ol )3 [1234, 1233], [Th(OPr )4]B [106], Er(OPri)j arROH [1734], and so on. The properties of the polymeric and tetrameric samples of Ti(OMe)4 ( A and B forms) [866] that differ in solubility in hydrocarbons, structure, and IR spectra are compared in Section 12.11. Dioxomolybdates and tungstates M02(0R)2 are also prone to polymerization in time their solubility decreases to practically 

On addition of PrOH to the crystals it is possible first to obtain very concentrated solutions, from which then commences the crystallization of poorly soluble tetrameric form (Fig. 6.2). It can be supposed that the trimer molecule coordinates an additional alcohol molecule along the sixth coordinate of the central aluminum metal in solution and then forms with the aid of hydrogen bonding the intermediates, which transform subsequently into tetrameric molecules already containing the hcxacoordi nated central A1 atom. The incomplete coordination polyhedron ofthe A1 atom is thus responsible for the thermodynamic instability ofthe phase, where it is present [1642], 

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