2 Homometallic Alkoxides

Homometallic Alkoxides 73
affected in these derivatives as the branching of alkoxo groups increases. 503 Thus
along with dialkyltin disopropoxides and di-tert-alkoxides, the normal alkoxides are
also monomeric in dilute solution, 262,311,504 but the latter derivatives show some association
in concentrated solution. 262,311 Cryoscopic data show that dibutyltin dimethoxide
is dimeric in character. 504
Apart from the smaller sizes of silicon and germanium atoms, the monomeric nature
of silicon and germanium alkoxides in contrast to analogous tin derivatives may be
ascribed to stronger p –d bonding in the former which diminishes in germanium and
appears to be insignificant in case of tin alkoxides. It may be assumed that the covalent
bonding in the case of silicon would be stabilized more by p –d bonding rather than
by bridging ( -bonding) and the shortened bond length in silicon methoxide has been
observed experimentally. 505
3.2.13 Alkoxides of Group 15 Elements
The alkoxides of arsenic and antimony are highly volatile, generally distil easily at
low temperatures, and exhibit monomeric behaviour. 244,506,507
The first homoleptic bismuth trialkoxides, Bi(OR)3 (R D Me, Et, Pr i ), prepared by
Mehrotra and Rai 202 in 1966, showed very limited solubility (in benzene or toluene)
and low volatility probably became of the formation of polymeric networks involving
alkoxo bridges. By contrast, Bi(OBu t )3 is highly soluble (even in hexane) 203–205 and
exhibits higher volatility (sublimed at 80–1000 Ž C/0.01 mmHg) 204,205 and monomeric
nature. The derivative Bi(OCH2CH2OMe)3 is dimeric in solution although in the
solid state a polymeric structure has been established by X-ray crystallography. 203–205
Interestingly, the derivative Bi(OCMe2CH2OMe)3 is volatile and appears to be
monomeric. 340,508
3.2.14 Alkoxides of Group 16 Elements (Se, Te)
Selenium and tellurium alkoxides are also highly volatile and tend to be
monomeric. 208,509,510 For example, ethoxo-, n-butoxo-, and isopropoxo-derivatives
of selenium have been volatilized at 76 Ž C/10.0, 110 Ž C/7.0, and 88 Ž C/4.5 mm
pressures, respectively. 510 Similarly tellurium tetramethoxide, ethoxide, n-butoxide,
and isopropoxide 208 distilled at 115 Ž C/9.0, 107 Ž C/5.0, 150 Ž C/0.8, and 76 Ž C/0.5 mm
pressure, respectively.
3.3 Infrared Spectra
Infrared spectroscopy has been utilized to verify the identity of metal alkoxides by
observing bands (M–O andC–O stretching vibrations) characteristic of the bonded
alkoxide group. Owing to the complex structure and often low molecular symmetry
of metal alkoxides, the assignment of various types of M–O bands has proved to be
rather difficult and the infrared technique has not generally been definitive in structural
assignments.
Barraclough et al. 511 studied the infrared spectra of a number of metal (aluminium,
titanium, zirconium, hafnium, niobium, and tantalum) alkoxides and tentatively
assigned the (C–O)M and (M–O) bands in these derivatives. It has been observed