Chemistry Reference
In-Depth Information
5
Metal Oxo-alkoxides
1
INTRODUCTION
Metal oxo-alkoxides [MO
x
OR
y
2
x
]
z
may be regarded as intermediates between the
oligomeric metal alkoxides [M
OR
y
]
n
x
D 0
and the three-dimensional macromolec-
ular metal oxides [MO
y
/
2
](
x
D
y
/
2;
y
D oxidation state of M). Indeed most metal
alkoxides
are readily converted by
hydrolysis
to the
corresponding metal oxide
(Eq. 5.1).
M
OR
y
C
y
2
H
2
O ! MO
y
/
2
C
y
ROH
5
.
1
For many years this moisture sensitivity of metal alkoxides caused problems in the
synthesis and characterization of pure metal alkoxides. Special techniques were devel-
oped for the elimination of water from reactants, solvents, and apparatus, thereby
enabling not only the synthesis of metal alkoxides but also their characterization
by infrared spectra, NMR spectra, mass spectra, and single-crystal X-ray diffraction
analysis.
Apart from the pioneering researches of Bradley and co-workers,
1
who studied the
controlled hydrolysis of the alkoxides of some early transition metals, there was little
interest in the metal oxo-alkoxides until the sol - gel technique was developed for the
conversion of metal alkoxide precursors into metal oxides and heterometal oxides
(Chapter 7).
2
,
3
A concise review of the chemistry of metal oxo-alkoxides published by
Mehrotra and Singh
4
reveals the rapid growth of interest in this field. The most notable
feature has been the dramatic increase in the number of crystal structure determinations
in recent years. However, our knowledge of the mechanism of formation of metal
oxo-alkoxides leaves much to be desired.
2
SYNTHETIC METHODS
Although the hydrolysis of a metal alkoxide is the obvious route to the formation
of a metal oxo-alkoxide other methods have been identified and these will also be
mentioned.
2.1
Hydrolysis
It is presumed that the first stage in the hydrolysis of a metal alkoxide is the formation
of a hydroxo derivative which then condenses to form the oxo ligand (Eqs 5.2 and 5.3).
