2 Homometallic Alkoxides

Homometallic Alkoxides 95
ligand field parameter (10 Dq D 15 000 cm 1 ) and the above spectral data accorded
with an octahedral configuration for the chloride alkoxides.
The electronic reflectance spectra592a of the chloride alkoxides of d2 vanadium(III):
VCl⊲OMe⊳2,VCl⊲OMe⊳2MeOH, VCl2⊲OMe⊳.2MeOH exhibited signals around 16 000,
25 000 and 35 700 cm 1 whichwereassignedto3 3
T2g T1g⊲F⊳, 3 3 T1g⊲P⊳ T1g⊲F⊳
3T1g⊲F⊳ transitions, respectively with the Racah parameter, B D 745 cm 1
and 3 A2g
and the ligand field parameter 10 Dq D 17 400 cm 1 . These spectra also showed a very
weak shoulder at 10 000 cm 1 which was assigned to the spin-forbidden transitions,
1T2g, 1 3
Eg T1g⊲F⊳. The above observations were well in accordance with the octahedral
geometry observed for various vanadium(III) complexes. 593,594
The reflectance spectra of d3 chromium(III) methoxide and ethoxide219,547 agreed with
ligand field predictions for an octahedral d3 ion with 10 Dq ¾ 17 000 cm 1 (Table 2.22).
The reflectance spectrum (transitions at 18 200 and 22 200 cm 1 )ofd4 Cr⊲OMe⊳2
showed evidence for a tetragonally distorted octahedral configuration219 for chromium(II).
The reflectance spectrum of d5 manganese dimethoxide219 showed spin-forbidden
transitions at 18 500, 24 400, 27 070, 29 000, and 31 300 cm 1 consistent with an octahedral
geometry around Mn(II). Thus a polymeric methoxo-bridged octahedral structure
was envisaged.
The high-spin d6 iron dimethoxide gave a band at around 10 000 cm 1 in its reflectance
spectrum which was assumed to arise from the 5 5
Eg T2g transition required for an
octahedral iron(II) ion, 219 and it probably attains a polymeric methoxo-bridged
edge-sharing octahedral structure. The diffuse reflectance spectrum of solid d5 iron
trimethoxide474 showed two sharp bands at 5800 and 7200 cm 1 and a broad band
with a maximum at 11 000 cm 1 . The bands observed at 5800 and 7200 cm 1 have
been assumed to be a mixture of the overtones of the C–H stretching and bending
modes, whereas that at 11 000 cm 1 was assigned to a spin-forbidden transition of the
tetrahedrally coordinated iron(III) ion.
The electronic spectrum of high-spin d7 cobalt dimethoxide exhibited bands
at 9500, 12 000, 17 900, and 21 000 cm 1 . The band at 9500 cm 1 was assigned
to 4 4 T2g⊲F⊳ T1g⊲F⊳ transitions, that at 12 000 cm 1 to 2 4 Eg⊲G⊳ T1g, and
4 4 A2g⊲F⊳ T1g transitions whereas the latter two bands corresponded to
4 4 A2g⊲F⊳ T1g and 4 4 T1g⊲P⊳ T1g transitions, respectively. 219 The spectrum was
Table 2.22 Electronic spectral data (cm −1 )forchromium(III)
alkoxides prepared by different routes 7
Compound 4 T2g 4 A2g (10 Dq) 4 T1g 4 A2g B
Cr⊲OMe⊳3 1 17 610 24 150 612
Cr⊲OEt⊳3 1 17 000 23 470 600
Cr⊲OBu t ⊳3 1 17 060 23 700 610
Cr⊲OMe⊳3 2 17 240 23 800 609
Cr⊲OEt⊳3 2 16 370 23 470 654
Cr⊲OPr i ⊳3 2 15 880 22 940 720
Cr⊲OMe⊳3 3 17 180 24 150 613
Cr⊲OPr i ⊳3 3 16 180 23 280 728
1 Prepared by lithium alkoxide method; 2 Prepared from Cr⊲OC6H5⊳3; 3 Prepared
by alcoholysis of Cr⊲OBu t ⊳4.