Chemistry Reference
In-Depth Information
Table 2.18
Assignment of M-O and C-O stretching frequencies in metal ethoxides,
[M(OEt)
x
]
n
M
Al(
III
)
1
f(
IV
)
1
H
U(
VI
)
Ti(
IV
)
r(
IV
)
Nb(
V
)
a(
V
)
(
V
)
-
-
1176 (s)
-
1160 (s)
1168 (vs)
-
-
-
-
-
-
-
1145 (s)
1145 (s)
1143 (sh)
1148 (sh)
-
Terminal C - O
-
-
-
1138 (s)
1134 (s)
-
-
-
1126 (w)
vibrations
1089 (m) 1089 (vs) 1098 (m) 1103 (s)
1095 (m)
1094 (m)
11110 (s)
1120 (s)
1097 (s)
1049 (s)
1044 (vs) 1079 (m) 1064 (s)
1073 (m)
1075 (m)
1066 (s)
1070 (s)
1056 (s)
880 (m)
899 (s)
930 (m)
913 (m)
918 (m)
920 (m)
914 (m)
917 (m)
905 (m)
Bridging C - O
-
-
1053 (vs) 1040 (m) 1050 (m)
1039 (s)
1030 (m) 1028 (m) 1024 (m)
vibrations
-
-
895 (s)
882 (m)
894 (m)
878 (m)
880 (w)
878 (w)
874 (m)
520 (m)
1
500 (m)
1
Terminal M - O
-
513 (m)
703 (m)
623 (m)
575 (m)
554 (m)
502 (m)
412 (m)
1
vibrations
392 (m)
512 (m)
520 (m)
425 (m)
-
-
-
463 (m)
1
470 (m)
1
Bridging M - O
-
-
646 (m)
577 (m)
485 (m)
499 (m)
390 (m)
vibrations
1
Nujol mull; all other spectra in CS
2
solution.
bridging groups by coordinated pyridine:
M
2
OEt
10
C 2C
5
H
5
N ! 2[C
5
H
5
N ! M
OEt
5
]
2
.
179
For example the bands at 1030, 880, and 485 cm
1
in Nb
2
(OEt)
10
and at 1028, 878,
and 499 cm
1
in Ta
2
(OEt)
10
were all eliminated by the addition of pyridine, thus
confirming the assignments in Table 2.18. Interestingly the terminal M-O bands at
575 and 554 cm
1
in Nb
2
(OEt)
10
and Ta
2
(OEt)
10
, respectively, were not diminished in
intensity but they were shifted to lower frequencies and to 548 cm
1
, respectively, on
coordination with pyridine.
In a further independent check on the assignments, a sample of
18
O-labelled (72%)
Ta
2
(OEt)
10
was synthesized from Et
18
OH as follows:
2Ta
NMe
2
5
C 10Et
18
OH ! Ta
2
18
OEt
10
C 10Me
2
NH
2
.
180
A comparison of the spectra of Ta
2
(
16
OEt)
10
and Ta
2
(
18
OEt)
10
should reveal significant
shifts in the frequencies of C-OandM-O vibrations but relatively smaller changes
for other bands. The data are presented in Table 2.19.
Using a simplified “diatomic molecule” approach for the C-OandM-O vibrations,
values were calculated for the frequencies of C-
18
OandM-
18
O vibrations and in
Table 2.19
Oxygen isotope effect on the spectrum of Ta
2
(OEt)
10
Ta(
18
OEt)
10
Ta
2
(
18
OEt)
10
Ta
2
(
16
OEt)
10
(observed)
(calculated)
C-O(cm
1
)
1148
¾1118
1120
1120
1098
1092
1070
1061
1044
1028
1018
1003
917
906
895
878
863
857
M-O(cm
1
)
554
542
525 (544)
499
486
473 (490)
