2 Homometallic Alkoxides

74 Alkoxo and Aryloxo Derivatives of Metals
that (C–O)M bands generally appear in the region 900–1150 cm 1 and the position
of bands is determined by the nature of the alkoxo groups. For example, the ethoxide
derivatives show two bands around 1025 and 1070 cm 1 which have been assigned
to s(C–O)M and as(C–O)M stretching vibrations, respectively. However, C–O and
C–C coupling complicates the problem. On the other hand isopropoxide derivatives
show bands at 1170, 1150, and 950 cm 1 . The isopropoxide derivatives also exhibit a
strong doublet at about 1375 and 1365 cm 1 due to gem dimethyl groups. Bell et al. 512
had also observed isopropoxide bands at about 1170, 1135 and 1120 cm 1 in boron
and aluminium isopropoxides.
Guertin et al. 513 had earlier assigned the band above 1000 cm 1 in aluminium alkoxides
to the (Al–O) stretching vibration. This observation was later corrected by Barraclough
et al. 511 who suggested that the band above 1000 cm 1 was due to (C–O) and a
set of five bands observed in the range 539–699 cm 1 were due to (Al–O) stretching.
Wilhoit et al. 493 also measured the infrared spectra of pure aluminium ethoxide as well
as the hydrolysed product and concluded that on hydrolysis the intensities of infrared
bands diminished to a considerable extent except the strong O–H andAl–O–Al bands
observed at 3340 and 935 cm 1 , respectively. Maijs et al. 514 measured the refraction
of Al–O bonds of bridging as well as terminal alkoxo groups in aluminium alkoxides
and the values derived were 0.5 and 2.5, respectively. The presence of five bands due
to Al–O bonds was ascribed to the presence of terminal and bridging groups in these
polymeric alkoxides.
Thus the (C–O) bands in the metal alkoxides (methoxides, ethoxides, isopropoxides,
n-butoxides and tert-butoxides) appear at 1070, 1025; 1070, 1025; 1375, 1365, 1170,
1150, 980, 950; 1090, 1025; and 940 cm 1 , respectively.
The infrared and Raman spectra of group 14 metal alkoxides have been studied
in detail by several workers and the M(C–O) and (M–O) bands have been firmly
assigned. For example, as(Si–O) and s(Si–O) bands in silicon tetra-alkoxides appear
in the ranges 720–880 and 640–780 cm 1 , respectively. 515–518 The alkyl group appears
to influence the (Si–O) band position in these silicon alkoxides; thus the bands shift to
higher values on increasing the size of the alkoxo groups. 516,518 Newton and Rochow 519
have investigated the infrared spectra of a variety of trialkoxosilanes (RO)3SiH and
the bands observed in the range 840–880 cm 1 have been assigned to as(Si–O)
stretching modes and the very weak band observed due to s(Si–O) in the infrared
spectrum gave a highly polarized band in Raman spectra at 700 cm 1 . The infrared
and Raman spectra of some alkylsilicon alkoxides, phenoxides, and chloride-alkoxides
have also been measured and the change in position of (Si–O) stretching modes
caused by substituents have been interpreted to be due to a change in polarity of
-bonds. 520–530
Infrared spectra of germanium tetra-alkoxides showed bands at 1040 and 680 cm 1
due to GeO4 configuration. 531 Studies 532,533 on the spectra of alkyl- and aryl-germanium
alkoxides indicate the presence of (Ge–O) bands in the range 840–500 cm 1 .
The spectra of a variety of alkyltin alkoxides have been measured by several
workers 210,265,534–540 and the band around 500 cm 1 in these derivatives was tentatively
assigned to (Sn–O) stretching modes. The bands observed at about 700 and
670 cm 1 have been assigned to (Sn–CH2) rocking vibrations arising from gauche
and trans conformations, respectively. The bands due to s and as(Sn–C) stretching
vibrations, respectively, are at about 500 and 600 cm 1 . The strong bands observed in