2 Homometallic Alkoxides

4 Alkoxo and Aryloxo Derivatives of Metals
type reaction(s) (Eq. 2.1):
M—OCHR 0 R 00 ! M—H C R 0 R 00 CDO
#
M C 1
2 H2
⊲2.1⊳
These, therefore, are not generally isolable under ambient conditions unless special
types of chelating alkoxo ligands 21 are used.
Although single crystal X-ray studies presented considerable difficulties in the earlier
stages, 25 the development of more sophisticated X-ray diffraction techniques has led
to the structural elucidation of a number of homo- and heteroleptic alkoxides 17–23 and
actual identification of many interesting metal oxo-alkoxide systems (Chapter 4).
In this chapter we shall discuss the synthesis, 3,4,26 chemistry and properties of
homometallic alkoxides with more emphasis on homoleptic alkoxides [M(OR) x ]n and
M(OR) x .Ln with occasional references to metal oxo-alkoxides MOy⊲OR⊳⊲x 2y⊳ and
metal halide alkoxides M(OR) x yXy.Lz (where x D valency of metal, L D neutral
donor ligand, X D halide, and n, y and z are integers). The discussion will generally
exclude organometallic alkoxides and a considerable range of metal-organic compounds
containing alkoxo groups, as in these systems the alkoxo groups play only a subsidiary
role in determining the nature of the molecule.
2 METHODS OF SYNTHESIS
Metal alkoxides in general are highly moisture-sensitive. Stringent precautions are,
therefore, essential during their synthesis and handling; these involve drying of all
reagents, solvents, apparatus, and the environment above the reactants and products.
Provided that these precautions are taken, the preparation of metal alkoxides, although
sometimes tedious and time consuming, is relatively straightforward.
The method employed for the synthesis 3,4,8,17,21 of any metal/metalloid alkoxide
depends generally on the electronegativity of the element concerned. Highly electropositive
metals with valencies up to three (alkali metals, alkaline earth metals, and
lanthanides) react directly with alcohols liberating hydrogen and forming the corresponding
metal alkoxides. The reactions of alcohols with less electropositive metals
such as magnesium and aluminium, require a catalyst (I2 or HgCl 2 ) for successful
synthesis of their alkoxides. The electrochemical synthesis of metal alkoxides by anodic
dissolution of metals (Sc, Y, Ti, Zr, Nb, Ta, Fe, Co, Ni, Cu, Pb) and even metalloids
(Si, Ge) in dry alcohols in the presence of a conducting electrolyte (e.g. tetrabutylammonium
bromide) appears to offer a promising procedure (Section 2.2) of considerable
utility. It may be worthwhile to mention at this stage that the metal atom vapour technique,
which has shown exciting results in organometallics, may emerge as one of the
potential synthetic routes for metal alkoxides also in future.
For the synthesis of metalloid (B, Si) alkoxides, the method generally employed
consists of the reaction of their covalent halides (usually chlorides) with an appropriate
alcohol. However, the replacement of chloride by the alkoxo group(s) does not appear to
proceed to completion, when the central element is comparatively more electropositive.
In such cases (e.g. titanium, niobium, iron, lanthanides, thorium) excluding the strongly
electropositive s-block metals, the replacement of halide could in general be pushed