A solution and solid state study of niobium complexes University of ...

2.7 Alkoxides
Chapter 2
Specific knowledge of the nature of the species formed in alcoholic solutions of metal
ions is essential for the manufacture of the final product. Metal alkoxides have the
general formula M(OR)x, where M is the metal with a valence x and O is the oxygen
atom that connects the alkyl group (R) to the metal at each valence site. Metal
alkoxides are usually very reactive species due to the presence of the
electronegative alkoxy groups that make the metal atoms susceptible to nucleophilic
attack.
The properties of metal alkoxides are mainly determined by the shape and size of
the alkyl group (R) as well as by the stereochemistry, valency, atomic radius and
coordination number of the metal. Based on the high electronegativity of oxygen, the
M-OR bonds are expected to possess fundamental ionic character.
According to Bradley’s concept, 70 alkoxides with the lower primary or secondary alkyl
groups have a strong tendency towards polymerization, creating coordination
polymers [M(OR)x]n (where n is the degree of polymerization). The magnitude of
polymerization is dependent on the following factors: 71
Aggregation increases as the metal atom becomes more electron deficient.
The larger the size of the metal atom, the greater the tendency to increase the
degree of association by forming alkoxo bridges.
The steric effects of the alkyl substituents. They inhibit aggregation with an
increase in steric demand and have been found to be of greater importance
than the electronic nature of the substituents in determining the final extent of
aggregation.
2.7.1 Preparation of Metal alkoxides
Various methods are available for the preparation of metal alkoxides. The choice of
synthesis depends on the ionisation energy of the metal. The less electronegative
70 D. C. Bradley, Nature, 182, 1211, 1958.
71 D. C. Bradley, R. C. Mehrotra, D. P. Gaur, Metal Alkoxides, Academic Press, London, 1978.
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