2 Homometallic Alkoxides

Homometallic Alkoxides 105
The mass fragmentation patterns of gallium ethoxide (Fig. 2.11) appear to indicate
the presence of higher molecular weight species, pentamers as well as hexamers, in
addition to other lower species but in this case also dimeric species predominate over
the others.
Lead di-tert-butoxide which was X-ray crystallographically characterized to be a
trimer 339a has been shown by mass spectrometry 194 to be dimeric in the gas phase.
3.8 EXAFS and XANES Spectral Studies
There are several attractive features of the Extended X-ray Absorption Fine Structure
(EXAFS) technique 613–615 which make it a powerful structural tool; notably (i) it is
extremely fast, (ii) both sample preparation and data collection are relatively easy,
without the requirement of single crystals, (iii) being sensitive to short-range order in
atomic arrangements, it can focus on the local environment of specific absorbing atoms,
and (iv) the technique is useful for a wide variety of materials such as amorphous solids,
liquids, solutions, gases, polymers, and surfaces.
The usefulness of the EXAFS technique in providing structural information (bonding
sites and bond lengths) for metal alkoxides was demonstrated in 1988 by the work of
Sanchez and co-workers 616 on titanium tetra-alkoxides, [Ti⊲OR⊳4]n (R D Et, Pr i ,Bu n ,
Am t ). Titanium tetra-tert-amyloxide, [Ti⊲OAm t ⊳4] and isopropoxide, [Ti⊲OPr i ⊳4] were
shown to be tetrahedral monomeric (n D 1) molecules with Ti–O bond lengths of
1.81 and 1.76 ˚A, respectively, whereas the ethoxide, and n-butoxide, were shown to
be trimers (n D 3) with average Ti–O bond lengths of 1.80 (terminal) and 2.05 ˚A
(bridging), as well as Ti–Ti interactions (observed for the first time) of 3.1 ˚A. This
confirms the trimeric behaviour previously shown by molecular weight studies. 455
The X-ray Absorption Near Edge Structure (XANES) 615 spectral features have been
used as an additional method for elucidating the stereochemistry as well as the oxidation
state of the metal. 616 For example, based on the information available from XANES
studies on titanium tetra-alkoxides, 616 the most probable coordination state of titanium
in trimeric ethoxide and butoxide [Ti⊲OR⊳4]3 (R D Et, Bu n ) has been suggested to
be five.
Although the most direct structural tool is single-crystal X-ray crystallography, yet
in some cases (particularly those for which X-ray structures cannot be available) useful
complementary information can be obtained by techniques like EXAFS and XANES
in addition to those discussed in earlier sections.
4 CHEMICAL PROPERTIES
4.1 Introduction
As stated earlier the metal–oxygen bonds in metal and metalloid alkoxides [M⊲OR⊳x ]n
are polarized in the direction, M υC —O υ —C owing to higher electronegativity (3.5
on Pauling scale) of oxygen. Consequently the metal atoms are easily susceptible to
nucleophilic attack, not only on account of this positive charge, but also owing to the
presence of energetically suitable vacant orbitals which can accommodate electrons
from nucleophiles. In addition, the oxygen atoms are also susceptible to electrophilic