2 Homometallic Alkoxides

Metal Oxo-alkoxides 435
trigonal bipyramid) and the other two are five-coordinated (lone pair in the vacant
octahedral site). 24
The only copper oxo-alkoxide structure determined to date is the giant
trimethylsiloxo derivative [Cu18⊲ 4-O⊳2⊲OSiMe3⊳14] containg two-coordinated (linear)
copper(I). 181
5.9.2 Aluminium
The first structural report of an aluminium oxo-alkoxide concerned the mixedligand
complex [H5Al5⊲ 5-O⊳⊲ -OBu i ⊳8] containing the novel Al5O9 framework
with a square pyramidal array of aluminium atoms. The apical Al is octahedrally
coordinated by one terminal hydride trans to the 5-oxo ligand and connected to
the four basal Al atoms by -isobutoxo ligands. Each basal Al is five-coordinated
(TBP) by one terminal hydride in an axial position trans to the 5-oxo ligand
and three -isobutoxo bridges in the equatorial positions to the apical Al and two
adjacent basal aluminiums. 182 Several years later the corresponding oxo-isobutoxide
[⊲Bu i O⊳5Al5⊲ 5-O⊳⊲ -OBu i ⊳8] (Fig. 5.22) was isolated and characterized and shown to
have the same Al5O9 framework. 79 Meanwhile the scandium, yttrium, and lanthanide
species [⊲RO⊳5M5⊲ 5-O⊳⊲ 3-OR⊳4⊲ -OR⊳4] had been characterized in which all five
metals were six-coordinated. 16
A remarkable decanuclear oxo-ethoxide [Al10⊲ 4-O⊳2⊲ 3-O⊳2⊲ -OEt⊳14⊲OEt⊳8] was
reported which contained aluminium in four-, five-, and six-coordination (Fig. 5.23). 183
The average Al–O bond distance for the 4-oxo ligand was significantly longer
(1.880 ˚A) than for the 3-oxo ligand (1.801 ˚A). In the tetranuclear compound
[Al4⊲ 4-O⊳⊲ -OBu i ⊳5⊲OBu i ⊳5⊲Bu i OH⊳] the four aluminiums are in a distorted
tetrahedral configuration around the 4-oxo ligand (Fig. 5.24). 184 Although the
OH hydrogen in the isobutanol ligand was not located it was clearly present
Figure 5.22 Structure of [Al5⊲ 5-
O⊳⊲ -OBu i ⊳8⊲OBu i ⊳5] (ž D 5-oxo;
H atoms omitted).