2 Homometallic Alkoxides

54 Alkoxo and Aryloxo Derivatives of Metals
Table 2.3 Proposed correlation amongst empirical formula of a metal alkoxide,
coordination number, geometry, and molecular association 432
Metal alkoxides Coordination Stereochemistry Minimum degree
M(OR)x number of M of M of association
MOR 2 OMO D 180 Ž 2
2 OMO D 120 Ž 3
3 Pyramidal 4
M(OR)2 3 Planar One OMO D 90 Ž 2
3 Planar, OMO D 120 Ž 3
4 Tetrahedral 3
4 Planar 4
6 Octahedral Infinite
three-dimensional
polymer
M(OR)3 4 Tetrahedral 2
4 Planar 2
4 and 6 Tetrahedral and octahedral 4
6 Octahedral 8
M(OR)4 5 Trigonal bipyramidal 2
6 Octahedral 3
8 Cubic 4
M(OR)5 6 Octahedral 2
8 Cubic 4
M(OR)6 8 Cubic 2
configuration for transition metals, leading to pronounced ligand field effects or/and
metal–metal bonding. 359
The data in Table 2.3 show that the values of molecular association predicted by
the simple theory of Bradley 432 are in reasonable agreement with those observed
experimentally. When the bulk of information collected from spectroscopic and other
physicochemical measurements is combined with Bradley’s structural theory as well
as with the knowledge of structures obtained from the more recent X-ray crystallographic
studies, a general classification of the possible structural types (Fig. 2.1)
becomes feasible. However, it is not still possible to predict precisely the structure for
any given metal alkoxide, and sometimes more than one structural type is possible.
The applications of physicochemical techniques such as IR, NMR, ESR, UV-Vis,
and mass spectroscopy as well as magnetic susceptibility measurements have played
an important role in the development of metal alkoxide chemistry, by throwing clearer
light in the absence of X-ray crystallographic data on the structural features of metal
alkoxides up to the 1980s. In spite of some inherent difficulties in the X-ray structural
elucidation of metal alkoxide species as pointed out by Bradley (25), the availability
of more sophisticated X-ray facilities during the last decade has played a key role in
providing solid state structural information on a rapidly increasing number of novel
homo- and heterometallic alkoxide systems.
An account of the physical characteristics of homometallic alkoxides is now presented
under the following headings:
1. Molecular complexities, volatilities, and thermodynamic data (Section 3.2)
2. Infrared spectra (Section 3.3)