2 Homometallic Alkoxides

X-Ray Crystal Structures of Alkoxo Metal Compounds 233
R
O
O
R O
R
M M
R
O R
O
O
R
Figure 4.8 Structure
of [M2⊲OR⊳6].
support of bridging alkoxo ligands 22 (Fig. 4.8). Chisholm and co-workers have established
a fascinating branch of alkoxide chemistry based on the chemical reactivity of
these molecules.
Notwithstanding attempts to rationalize the many known structures of metal alkoxides
it remains problematical at best to predict any detailed structures. It is not clear,
for example, why tetranuclear titanium tetra-alkoxides adopt the structure shown in
Fig. 4.4 in preference to the alternative cubane structure [M4⊲ 3-OR⊳4(OR) 12]inwhich
the facial configuration of three nonbridging alkoxo groups on each metal enjoys the
enhanced -bonding by virtue of being trans to the non- -bonding 3-alkoxo groups.
2 STRUCTURES OF ALKOXO COMPOUNDS OF METALS
2.1 Structures of Alkali Metal Alkoxides
2.1.1 Lithium
Lithium methoxide forms a sheet polymer [LiOMe] n in which the lithiums are fourcoordinated
by 4-OMe groups 23 but with more sterically hindered groups a hexanuclear
species [LiOCMe2Ph]6 (Fig. 4.9) is formed involving 3-coordinated Li with 3-alkoxo
groups. 24 With excessively bulky alkoxo groups, dimeric molecules [Li⊲ -OCBu t 3 ⊳]2 are
Figure 4.9 Li6⊲ 3-OC⊳6 core
structure of [Li6( 3-OR)6].