2 Homometallic Alkoxides

72 Alkoxo and Aryloxo Derivatives of Metals
gallium isopropoxide and tert-butoxide to be dimeric in boiling benzene. The dimeric
behaviour of gallium isopropoxide was confirmed in a later study by Oliver and
Worrall 495 who found it to be dimeric cryoscopically in dilute benzene solution.
However, the cryoscopic molecular weight determination of a freshly prepared
concentrated benzene solution of the isopropoxide indicated it to be tetrameric.
Evidence in favour of the latter observations was obtained from 1 H NMR spectra, which
also showed the presence of tetrameric species in a fresh solution which dissociated
to the dimeric form on being aged.
3.2.12 Alkoxides of Group 14 Elements
Silicon and germanium alkoxides are highly volatile and most of the derivatives
of silicon 316 have been distilled unchanged at atmospheric pressure; for example,
silicon tetraethoxide, 316,496 and tetraphenoxide 316,497 distilled at 168 Ž C and 417–420 Ž C,
respectively. Similarly, germanium ethoxide 315 and phenoxide 293 could also be
volatilized at 85–86 Ž C/12.0 mm and 220–225 Ž C/0.3–0.4 mm, respectively. The low
boiling points of these alkoxide derivatives of silicon and germanium are due to their
monomeric nature, irrespective of the chain length and branching of alkoxo groups.
Bradley et al. 113,273,274,432 measured the boiling points of a number of derivatives
under different pressures and observed that the results follow the equation, log p D a
b/T in the pressure range 2.0–10.0 mm. Although silicon alkoxides are monomeric,
the latent heat of vaporization, entropy of vaporization, and Trouton constants increase
with the length of the alkyl groups. This unexpected behaviour has been ascribed
to the molecular entanglement in the liquid state or to the restricted rotation of the
alkoxo group with increasing chain length. It is worth mentioning here that a molecule
in the liquid state will require special orientation in order to be disentangled before
vaporization, and this may cause an increase in the entropy of vaporization.
The thermal decomposition of silicon tetraethoxide yields silicon oxides at higher
temparatures. 475,498 By contrast, tin alkoxides appear to be thermally less stable than
the corresponding silicon and germanium analogues. 311,499–501 For example, all normal
alkoxides of tin undergo decomposition on being heated under reduced pressure. 311
The sterically hindered isopropoxide, tert-butoxide, and tert-amyloxide, however, were
distilled at 131 Ž C/1.6, 99 Ž C/4.0, and 102 Ž C/0.2 mm, respectively. 311 The ebullioscopic
molecular weight measurements in boiling benzene indicate the following order of
complexities: methoxide (an insoluble derivative) appears to be highly associated; the
normal alkoxides are tetrameric, whereas isopropoxide and tert-alkoxides are trimeric
and monomeric, respectively. 311
The alkyl substituents at the tin atom, in view of its larger size have been found to
affect the molecular complexities as well as volatility of the derivatives considerably.
For example, Gaur et al. 210,211 observed that except for highly associated nonvolatile
methoxides, all secondary and tertiary trialkoxides of monoalkyltin trialkoxides are
highly volatile liquids which could be distilled in good yields under reduced pressure.
The complexities of ethoxides, isopropoxides, and tert-butoxides of monoalkyltin
fall in the order 3.0, 1.5, and 1.0, respectively in boiling benzene. These molecular
complexities are in agreement with the 119 Sn NMR spectra measured for a number of
monoalkyltin trialkoxides. 502 Similarly, dialkyltin dialkoxides and trialkyltin alkoxides
also show high volatility but the molecular complexity is comparatively less