2 Homometallic Alkoxides

Homometallic Alkoxides 97
susceptibility of chromium tetra-tert-butoxide which was shown to be paramagnetic
in behaviour having a magnetic moment of 2.8 š 0.3 B.M. as required for a d 2 metal
ion. It was shown that the magnetic susceptibility followed the Curie–Weiss law and
was independent of temperature.
A detailed study of the magnetic susceptibilities of the methoxides of first row
divalent and trivalent transition metals has been published by Winter and coworkers
219,220,474,541,542,592,595,598 in a series of papers and these results along with
reflectance spectral data have been used to predict the stereochemistry of these
methoxides. The results are summarized in Table 2.23.
Vanadium trimethoxide 589 is also a paramagnetic compound having magnetic
moments 1.70–1.79 B.M. in the temperature range 150 to 16 Ž C. The data appear to
indicate a slight variation of the magnetic moment with temperature, and this magnetic
behaviour is well in accordance with the distorted octahedral configuration for the
trimeric vanadium trimethoxide. Similarly vanadium monochloride dimethoxide shows
temperature-dependent magnetic moments varying in the range 1.56–2.09 B.M. over
the 176 to 19 Ž C temperature range. These low values of magnetic moment have
been interpreted in terms of spin interaction between neighbouring metal ions within
the polymeric cluster.
Unlike d 2 vanadium trialkoxides, the d 1 tetra-alkoxides exhibit temperature independent
magnetic behaviour, e.g. dimeric tetramethoxide and monomeric tetra-tertbutoxide
follow the Curie law and both these compounds have magnetic moments
of 1.69 B.M. For the tetraethoxide, the magnetic and electronic spectrum measurements
correspond to five-coordinated vanadium having a trigonal bipyramidal structure
involving C2v symmetry in which dimerization occurs through an edge-sharing alkoxide
Table 2.23 Magnetic moment, reflectance spectral data, and proposed stereochemistry
of first row transition metal methoxides and ethoxides
Metal Magnetic moment Reflectance spectra 10 3 Proposed
alkoxide Colour eff ⊲B.M.⊳ 1 ⊲cm 1 ⊳ tentative assignment stereochemistry 2
Cr⊲OMe⊳2 Purple 5.16 ⊲ D 160 Ž ⊳ 18.2 ⊲dxy ! d x 2 y 2⊳; CrO4 (Sp)
22.2 ⊲d z 2, dxy, dxy ! d x 2 y 2⊳
Mn⊲OMe⊳2 Pale pink 5.96 ⊲ D 35 Ž ⊳ 18.5 ⊲ 4 T1g, G⊳; 24.4 ⊲ 4 Eg, G⊳; MnO6⊲Oh⊳
20.07 ⊲ 4 T2g, D⊳; 29.0 ⊲ 4 Eg, D⊳;
31.3 ⊲ 4 T1g, P⊳
Fe⊲OMe⊳2 Dark green 5.14 ⊲ D 0⊳ 10.0 ⊲ 5 Eg, D⊳ FeO6⊲Oh⊳
Co⊲OMe⊳2 Pale purple 5.46 ⊲ D 15 Ž ⊳ 9.5 ⊲ 4 T2g, F⊳; 12.0 ⊲ 2 Eg⊳; CoO6⊲Oh⊳
17.9 ⊲ 4 A2g, F⊳; 21.0 ⊲ 4 T1g, P⊳
Ni⊲OMe⊳2 Pale green 3.38 ⊲ D 0⊳ 8.7 ⊲ 3 T2g⊳; 14.5 ⊲ 3 T1g, F⊳; NiO6⊲Oh⊳
25.0 ⊲ 3 T1g, P⊳
Cu⊲OMe⊳2 Royal blue 1.24 ⊲22 Ž C, 15.6 ⊲dxy ! d x 2 y 2⊳; CuO4⊲Sp⊳
D 0⊳ 18.5 ⊲d z 2,dxy, dyz ! d x 2 y 2)
Ti⊲OMe⊳3 Yellow green Diamagnetic 10.0 —
Cr⊲OEt⊳3 Pale green 3.56 ⊲ D 270 Ž ⊳ 16.4 ⊲ 4 T2g⊳; 25.0 ⊲ 4 T1g⊳; CrO6⊲Oh⊳
38.1 ⊲ 4 T1g⊳
Fe⊲OMe⊳3 Yellow 5.86 ⊲ D 200 Ž ⊳ 11.1 FeO4⊲Td⊳
1
eff D 2.84⊲T C ⊳ 1/2 ; 2 Sp D square planar, Oh D octahedral, Td D tetrahedral.