Metal alkoxides homometallic

The reaction of metal halides with an alkoxide or aryloxide (most commonly an alkali metal alkoxide), can resnlt in metathetic exchange to give the required complex (equation 6). Such reactions can also result in the formation of heterometallic species (such as NaZr2(OR)4) this can be a problem when homometallic aUcoxides are desired, but it is an important route to heterometallic alkoxides and aryloxides. The nature of the product can be influenced by the alkali metal, the alkoxide ligand, and the relative amounts of the reactantsd ... 

In view of the solubility of metal alkoxides (binary or homometallic as well... 

Both homometallic and heterometallic oxo-alkoxides can be produced by partial hydrolysis of metal alkoxide precursors via dehydroxylation and dealkoxolation of metal hydroxo alkoxides. The crystal structures of a number of polynuclear titanium oxo alkoxides have been elucidated, these structures having evolved during the slow addition of small amounts of water to Ti(OEt)4 47,48,76-79... 

One major problem in producing gels containing homogeneous mixtures of a variety of oxides is that the precursors may not all hydrolyze at the same rate. In particular, transition metal alkoxides hydrolyze much more rapidly than silicon alkoxides. The controlled hydrolysis of low-molecular-weight homometallic species described in the previous section can be adapted to prepare mixed alkoxides. For example, pre-hydrolysis of metal alkoxide followed by reaction with the silicon alkoxide gives a mixed dimeric species such as ... 

Mixed-metal alkoxides are generally covalent [4]. As a result, they are soluble in organic nonpolar solvents (often less in alcohols), generally more than the parent alkoxides. Some can be sublimed without disproportionation, the volatility following the same variation as for the homometallic alkoxides (OtBu > OiPr > OEt > OMe). Mass spectrometry, for instance, established that heterometallic LiNb fragments were retained in the gas phase for LiNb(OEt)6. 

Information on the hydrolysis of mixed-metal alkoxides is even scarcer than for the homometallic. Partial hydrolysis of [LiNb(OEt)6]< leads to the dimeric heterometallic alkoxide [LiNbO(OEt)4(EtOH)]2, in which both the stoichiometry between the two metals and their coordination numbers are retained [93]. On the other hand, the partial hydrolysis of solutions of methoxyethoxides of Ba and Ti (1 1 molar ratio) offers Ba4Tii30i8(0C2H40Me)24 (30% yield). Its structure corresponds to a tetrahedron of BaOs units surimposed on a Ti06(Ti03)i2 core this Tii3042 core is related to that of the aluminum salt [A1i304(0H)24(H20)]2+ [83]. The important modification of the stoichiometry between the two metals illustrates the complexity of the sol-gel chemistry of multimetallic systems and shows that important structural rearrangements can occur. 

Interaction of Metal Complexes with Organic Ligands and Metal Alkoxides or Chemical Modiflcation of Complexes in Solutions (Method 2.5). This approach provides an important alternative to the Method 2.2 in the preparation of derivatives of late transition elements (the homometallic alkoxides of those being insoluble and not reactive polymeric solids). The reaction stoichiometry and conditions are dependent on the nature of reactants and on the composition of the product to be obtained. In some cases the reaction is facile and provides the desired products as the result of mixing the reactants in proper ratio (usually in toluene) and subjecting them to short reflux, for example (Boul-maaz, 1997 Kessler, 2003) ... 

Oxo-alkoxides are partially condensed species and represent a new class of sol-gel precursors. These species are oligomeric and can be either homometallic or heterometallic. In many cases the oxo bridges produced in these compounds are multiply bridging, in order to achieve the most favored coordination number of six for metal atoms in alkoxide species. Isolation of compounds with similar multiply bridging oxygen atoms has been observed by the reaction of metals with the appropriate phenol or alcohol, with, in some cases, reaction with solvent molecules producing compounds quite different from the simple binary alkoxides expected.72 Besides isolation of intermediates, it is also possible to monitor the reactions by NMR and mass spectrometry 73... 

Table 2 Homometallic Alkoxides of Metals Involved im Electroptical Ceramics ...

Addition reactions similar to Eq. (8) can also be observed between anhydrous acetates and alkoxides. Such reactions can even occur at room temperature, but they may require heating (depending on the lability of the metal acetate bond) and/or be less selective and give redistribution products [homometallic acetatoalkoxides, Af(OR)n-jc(OAc) ] as well. 

The chemical composition of the products is determined by a number of important factors, such as the nature of metal atoms involved, the nature of alkoxide groups, the ratio of homometallic reactants applied and, in certain cases, even on the solvent in which the reaction is carried out. For example, the reaction of barium ethoxide and titanium ethoxide in 1 (where x>2) ratio in oxygen-free solutions provides different products in alcohol and in hydrocarbon media (Yanovsky, 1995 Kessler, 1994) ... 

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