2 Homometallic Alkoxides

Metal Oxo-alkoxides 405
The hexanuclear [La2Mo4⊲O⊳4⊲ 4-O⊳4⊲ -OPr i ⊳8⊲OPr i ⊳6] has a tetragonal bipyramidal
configuration of metal atoms with the lanthanums trans to each other. Each Mo
atom has one terminal oxo ligand and the 4-oxo ligands are each bonded to two
La and two Mo atoms. The bridging isopropoxo groups span the LaMo edges of the
polyhedron and each metal has one terminal isopropoxo group. 116
5.3.2 Actinides
Among the actinides a few structures of uranium oxo-alkoxides have been reported.
The trinuclear [U3⊲ 3-O⊳⊲ 3-OBu t ⊳⊲ -OBu t ⊳3⊲OBu t ⊳6] uranium(IV) complex has
the familiar M3O11 core structure but there is no U ....U bonding in contrast
to [Mo3O⊲OCH2But⊳10], which exhibits metal–metal bonding. 117 The trinuclear
uranium(VI) compound [U3⊲O⊳2⊲ -O⊳2⊲ -OBu t ⊳2⊲OBu t ⊳8] has a central uranium
containing a linear UO2 group bridged by unsymmetrical -O oxo groups and -OBu t
groups to the outer U atoms. 64 The mononuclear [UO2⊲OBu t ⊳2⊲Ph3PO⊳2] has the linear
UO2 group and cis phosphine oxide ligands. 44
5.4 Titanium Sub-group Metal-containing Oxo-alkoxides (Including
Heterometallic Compounds) (Table 5.4)
5.4.1 Titanium
Numerous titanium oxo-alkoxides have been structurally characterized including
[Ti7O4⊲OEt⊳20], 9,118 [Ti10O8⊲OEt⊳24] 9 and [Ti16O16⊲OEt⊳32] 118 which with the
unhydrolysed crystalline tetrameric ethoxide [Ti4⊲OEt⊳16] conform to a general formula
[Ti⊲4C3x⊳O4x ⊲OEt⊳4⊲xC4⊳](x D 0, 1, 2, 4) implying an increment of Ti3O4⊲OEt⊳4 between
each member of the series. Interestingly this is very similar to the series proposed by
Bradley et al. 5 [Ti⊲3C3x⊳O4x ⊲OEt⊳4⊲xC3⊳] based on the analysis of the variation of number
average degree of polymerization with degree of hydrolysis of the tetraethoxide in boiling
ethanolic solution. The actual structures of the oxo-ethoxides obtained to date all involve
octahedra. In the heptanuclear [Ti7⊲ 4-O⊳2⊲ 3-O⊳2⊲ -OEt⊳8⊲OEt⊳12] the central TiO6
octahedron shares six edges through two cis- 4-oxo ligands, two trans- 3-oxo ligands,
and two cis- -ethoxo ligands to six other octahedra (Fig. 5.7). 118
Figure 5.7 Structure of [Ti7⊲ 4-
O⊳2⊲ 3-O⊳2⊲ -OEt⊳8⊲OEt⊳12]( D
4-oxo, D 3-oxo, ž D Ti; H
atoms omitted).