2 Homometallic Alkoxides

4
X-Ray Crystal Structures of Alkoxo
Metal Compounds
1 INTRODUCTION
Since 1980 an increasing number of single-crystal X-ray structure determinations has
been carried out and it is not feasible to describe more than a representative number
in detail. A number of reviews have been published. 1–5 In fact only a few homoleptic
metal alkoxides have been structurally characterized and the majority of structures
determined involve metal complexes containing other ligands besides the alkoxo group.
Therefore most of the structural data in this chapter are reported in tabular form.
The uninegative alkoxide ion RO contains a donor oxygen atom with three pairs
of electrons available for covalent bonding to a metal ion.
In the nonbridging mode the angle R � OM should reflect the degree of -donation in
the bonding (4-I–4-III).
R •• O
R
O
M M M
(4-I) (4-II) (4-III)
R •• O•
•
The extent of -bonding depends on the availability of vacant -type orbitals on the
metal ion.
Alkoxide bridges are a characterisic feature of metal alkoxide structures and in
principle are also affected by -bonding supplementing the -bonds (4-IV–4-VII).
R
O
M M
(4-IVa)
• •
R
O
M M
(4-VIa)
R
O
M M
(4-IVb)
• •
R
O
M M
(4-VIb)
R
O
M M
(4-V)
R
M
O
M
M
(4-VII)