2 Homometallic Alkoxides

210 Alkoxo and Aryloxo Derivatives of Metals
Absorbance
1.6
1.2
0.8
0.4
0.0
350
d
c
a
b
450
550
Wavelength (nm)
650 750
Figure 3.1 Spectra of [NifAl⊲OPr i ⊳4g2] in
(a) film, (b) C6H6, (c)Pr i OH, (d) C5H5N.
alcohol or pyridine solutions owing to the formation of bis-adducts. This change in geometry
is also reflected in a change of colour (pink in benzene solution or as thin film versus
yellowish green in isopropanol/pyridine solution). The octahedral form would obviously
be predominant in NifAl⊲OMe⊳4g2, but the tetrahedral form should be the predominant
species in NifAl⊲OPr i ⊳⊲OBu t ⊳3g2 and NifAl⊲OBu t ⊳4g2. 208
As mentioned at the end of Section 3.1, the tetra-alkoxoaluminates of some trivalent
metals (Sc, Y, Ln, 40,80,192 Cr(III), 105 and In 69,73 ) exhibit structures similar to that of
Alf⊲ -OPr i ⊳2Al⊲OPr i ⊳2g3, in which the central aluminium atom can be replaced by
other trivalent metals. These are again all monomeric and volatile, which suggests that
they are more stable than [Al⊲OPr i ⊳3]4, which distils with dissociation as a dimer.
Except for the tetraisopropoxoaluminates of cerium(IV) and thorium with
aluminium, diisopropoxozirconium bis-tetraisopropoxoaluminate derivatives of the
type [⊲Pr i O⊳2Al⊲ -OPr i ⊳2Zr⊲OPr i ⊳2⊲ -OPr i ⊳2Al⊲OPr i ⊳2] can only be obtained in
the case of smaller zirconium atoms. The structure of this derivative, those of
ZrAl2⊲OPr i ⊳10 x Lx (where L D acetylacetone, benzoylactone, dibenzoyl methane,
methylacetoacetate, or ethylacetoacetate), have been elucidated mainly by 1 HNMR
studies and can be represented 209,210 by
L
L
L
Al(m-OPr i )2Zr(m-OPr i )2Al
L
in which the terminal isopropoxo groups of the original derivative [⊲Pr i O⊳2Al⊲ -
OPr i ⊳2Zr⊲OPr i ⊳2⊲ -OPr i ⊳2Al⊲OPr i ⊳2] have been replaced by bidentate ˇ-diketone or
ˇ-ketoester ligands (L). It is interesting to record that the X-ray structure of the
L
L