2 Homometallic Alkoxides

230 Alkoxo and Aryloxo Derivatives of Metals
Thus, whereas modes (4-IV) and (4-VI) would imply symmetrical bridges, (4-V) should
be unsymmetrical. In addition to the all- -bonding 3-mode (4-VII) involving twoelectron
two-centred localized bonds, there is also the possibility of 4-bridging, etc.
involving delocalized bonding.
A distinguishing feature of the alkoxo group as a ligand is its steric effect. By
increasing the size and chain branching of the R-group the steric effect of RO may be
increased sufficiently to prevent alkoxo-bridging and thus result in a mononuclear metal
alkoxide. This effect is obviously greater in the alkoxides of metals in higher valencies,
e.g. M(OR) 6,M(OR) 5,M(OR) 4, owing to their greater intramolecular congestion. The
electronic effect of the alkoxo ligand also plays a part in structure determination. An
electronegative substituent X (e.g. CF3) inXCH2O– will reduce the electron density
on the donor oxygen thereby weakening the bridging modes (4-IV–4-VII). At the
same time the electrophilic nature of the metal ion will be enhanced, leading to the
acquisition of a supplementary neutral ligand L (e.g. THF, py, etc.) and the formation
of a mononuclear adduct [M(OCH2X)x Ly]. This is well illustrated by lanthanum which
forms the trinuclear tertiary butoxide [La3⊲ 3-OBu t )2⊲ -OBu t ⊳3(OBu t ⊳4(Bu t OH⊳2] 6 but
gives the mononuclear [LafOCMe(CF3⊳2g3(thf) 3] 7 with hexafluoro-tertiary butoxide,
although the metal is six-coordinated in both molecules.
In an early attempt to rationalize the known structures of homoleptic metal alkoxides
[M(OR) x]n it was noted by Bradley 8 that the alkoxides which were soluble in common
organic solvents usually exhibited low degrees of oligomerization (e.g. n D 2, 3, or 4). It
was proposed that in the less sterically demanding alkoxo groups (e.g. MeO, EtO, Pr n O,
etc.) the metal M utilized the bridging propensity of the alkoxo group to achieve the smallest
oligomer compatible with all the metal atoms attaining their preferred coordination
number. To minimize the complexity of a polynuclear metal alkoxide it is necessary to
maximize the number of bridging alkoxo groups between adjacent pairs of metal atoms.
For tetrahedral or octahedral metal coordination this means face-sharing rather than
edge-sharing polyhedra. Thus, although a trivalent metal can achieve four-coordination
(distorted tetrahedral) by forming an edge-bridged dimer [M2⊲ -OR) 2(OR) 4] (Fig. 4.1)
and a pentavalent metal can achieve six-coordination (distorted octahedral) by also forming
an edge-bridged dimer [M2⊲ -OR⊳2(OR) 8] (Fig. 4.2), a tetravalent metal would
require face-bridging octahedra in order to form the trimer [M3⊲ -OR⊳6(OR) 6] (Fig. 4.3),
the minimum sized oligomer for M(OR) 4. Molecular weight measurements on supercooled
titanium tetraethoxide in benzene solution indicated that a trimer was the predominant
species, but the X-ray crystal structures of [Ti4⊲OMe⊳16], [Ti4⊲OEt⊳16], and
[Ti4⊲OEt⊳4⊲OMe⊳12] all revealed the edge-sharing tetranuclear structure [Ti4⊲ 3-OR⊳2⊲ -
OR⊳4⊲OR⊳10] 9 (Fig. 4.4). It was observed by Chisholm 10 that this particular structure
is favoured by a number of species [M4⊲OR⊳x XyLz ](whereXD anionic ligand; L D
neutral ligand; x C y C z D 16). It thus appears that edge-sharing is favoured although
R
O
O
R
M
R
O M
O
R
R
O
O
R
Figure 4.1 Structure of
[M2⊲ -OR⊳2⊲OR⊳4].