2 Homometallic Alkoxides

Homometallic Alkoxides 75
the range 1030–1070 cm 1 due to (C–O) Sn in primary alkoxides shifted to a lower
value (940–980 cm 1 ) in the isopropoxides and tert-butoxides.
Infrared spectral studies on homoleptic methoxides and halide methoxides of the
first row divalent transition metals such as chromium, manganese, iron, cobalt, nickel,
copper, and zinc have been made by Winter and co-workers. 320,541–543 These workers
have also assigned the (C–O)M band in the region 1000–1100 cm 1 and the (M–O)
stretching frequencies fall below 600 cm 1 in these methoxides. For example the
(Ni–O) bands in nickel dimethoxide and halide-methoxides appear at 375, 420 cm 1 ,
and 360–390, 420 cm 1 , respectively 541,543 whereas in copper methoxides, (Cu–O)
bands appear at 520–545 and 410–450 cm 1 . 542
The earlier study on the infrared spectra of the transition metal alkoxides and the
tentative assignments made by Barraclough et al. 511 were later confirmed by Lynch
et al. 544 Table 2.17 indicates the positions of (C–O)M and (M–O) stretching modes
observed in these transition metal alkoxides.
Following the earlier work of Barraclough et al., Bradley and Westlake 545 made a
more detailed study of the infrared spectra of the polymeric metal ethoxides. In particular
they attempted a definitive assignment of bands to the C–O andM–O vibrations
of terminal and bridging ethoxide groups. Also, by measuring relative band intensities
of terminal and bridging groups, they attempted to assign structures to the polymeric
alkoxides. In assigning bands they used various criteria: thus by comparing the spectra
of [M(OEt)x]n with those of EtOH (no M–O vibrations and no bridging OEt groups)
and U(OEt)6 (an essentially monomeric compound containing no bands due to bridging
C–O andM–O groups), it was possible to identify the bands due to M–O andC–O
vibrations in terminal and bridging groups as listed in Table 2.18.
Owing to the structural complexity of these molecules and the possibility of coupling
of vibrations, the simple criterion of comparison mentioned above was not considered
an adequate basis for the assignment, and additional criteria were developed. For
example, it was shown that the addition of pyridine as a donor ligand caused the
disappearance of some bands but not others. It was reasonable to conclude that only
the bands due to bridging ethoxide groups would disappear, due to the replacement of
Table 2.17 Tentative assignments of n(M–O) stretching
frequencies in transition metal alkoxides
Metal alkoxide (C–O)M (cm 1 ) (M–O) (cm 1 )
Ti(OEt)4 1064, 1042 625, 500
Ti(OPr i )4 1005, 950 619
Ti(OAm t )4 1011 615, 576
Zr(OPr i )4 1011, 958, 945 559, 548
Zr(OBu t )4 997 540
Zr(OAm t )4 1010 586, 559, 521
Hf(OPr i )4 1020, 983 –
Hf(OBu t )4 990 567, 526
Th(OPr i )4 996, 973 –
Nb(OEt)5 1063, 1029 571
Ta(OEt)5 1072, 1030 556
Ta(OPr i )5 1001 557, 540