2 Homometallic Alkoxides

Homometallic Alkoxides 93
3.5 Electronic Absorption, Reflectance, and Electron Spin Resonance
Spectra
Electronic absorptions 585 may be broadly classified into charge-transfer and ligandfield
(d–d) bands. The latter are more helpful in providing information regarding the
probable geometries of metal alkoxide complexes. To interpret the spectra of metal
complexes in general in which the metal ions have more than one but less than nine
(d 2 –d 8 ) electrons, one is required to employ an energy level diagram based upon the
Russell-Saunders states of the relevant d n configuration in the free (uncomplexed) ion.
When d–d spectra are observed and assigned, it is possible to derive information on
the magnitude of the ligand field splitting in the derivatives, the spin–orbit coupling
constants and the Racah parameters, the latter providing a measure of interelectronic
repulsions.
The main group metal alkoxides are diamagnetic and do not show ESR spectra. The
paramagnetic transition metal complexes studied, however, have yielded information
concerning the delocalization of the unpaired electrons.
Titanium trimethoxide 219 was expected to have an octahedral environment around
Ti(III), but the reflectance spectrum gave a d–d transition at 10 000 cm 1 and the
compound was diamagnetic indicating a strong metal–metal interaction. The methanolsolvated
trimethoxide Ti⊲OMe⊳3⊲MeOH⊳6 shows a band at 25 200 cm 1586 which is
considerably higher than that observed for octahedral Ti⊲H2O⊳6 3C at 20 300 cm 1 . 587
For confirming the predicted stereochemistry of vanadium alkoxides, Bradley et al. 588
measured the ESR spectra of vanadium tetramethoxide in solid as well as solution
forms (using solvents like methylcyclohexane or benzene) at room temperature and
at 150 Ž C; broad signals were obtained at g D 1.955 š 0.005 and 1.94 š 0.01 in the
solid and the solution, respectively, without hyperfine structure. The spectra were in
accordance with the distorted octahedral geometry for trimeric species with 2 T2g state
split by >1000 cm 1 giving rise to an orbital singlet ground state. Solid vanadium
tetraethoxide also gave a very strong broad signal at room temperature (g D 1.945).
On the other hand the ESR spectra of the tetraethoxide in benzene, methylcyclohexane,
and carbon disulphide gave narrower signals in the range g D 1.951–1.953
with partial resolution of eight line 51 V interaction A0 D 78–79.1 gauss. 588 However,
the hyperfine structure disappeared at lower temperatures ( 170 Ž C). These results were
in good agreement with the dimeric five-coordinated (trigonal-bipyramidal) vanadium
tetraethoxide involving ethoxo bridges. The electronic spectra of solid and solutions
of vanadium tetraethoxide were found to be quite similar. 589 The spectra show two
d–d transition bands of rather weak intensities at 14 200 and 6000 cm 1 andanintense
charge-transfer band at 25 000 cm 1 . The band observed at 14 200 cm 1 ruled out the
possibility of involving a tetrahedral or octahedral vanadium(IV) atom, because it was
too high in energy and weak in intensity for a tetrahedral species and very low in
energy for an octahedral species. If the vanadium atom is five-coordinated, then the two
ligand field transitions are expected 1⊲ 2 E 0 2 E 00 ⊳ at 6000 cm 1 and 2⊲ 2 A 0 1 2 E 00 ⊳
at 14 200 cm 1 for two ligand field parameters Ds D 314 cm 1 and Dt D 1398 cm 1 .
These values are plausible for the trigonal bipyramidal structure expected for dimeric
five-coordinated vanadium tetraethoxide. However, although the values of Ds and Dt
are consistent with the D3h point group, the bridging ethoxide oxygen atoms should in
principle produce a different electric field from that of the terminal oxygen atoms, and