2 Homometallic Alkoxides

474 Alkoxo and Aryloxo Derivatives of Metals
changing the hybridization of the oxygen atom in a species X–O–Y from sp 3 to sp 2
and sp leads to bond angles of 109 Ž , 120 Ž and 180 Ž implies that this parameter may
be an accurate probe of -bonding in metal aryloxides. The field of metal aryloxide
chemistry grew dramatically with the use of sterically bulky phenoxides in order to
suppress oligomerization via aryloxide bridges and to control stoichiometry. Structural
studies showed early transition metal derivatives (group 4 and 5 elements) of these
ligands to possess very large (in some cases linear) M–O–Ar angles. 190 The large size
of these angles was initially attributed to steric factors both “bending away” the bulky
aryl group as well as fostering -bonding by maintaining a mononuclear environment
for the electrophilic metal centre. Even as early as 1966 Watenpaugh and Caughlin
had determined the crystal structure of dimeric [Cl2(PhO)Ti( -OPh)2Ti(OPh)Cl2] and
concluded that the short terminal Ti–OPh distance of 1.74 (1) ˚A was due to oxygen p to
metal d -bonding. 191 Furthermore the large Ti–O–Ph angle of 166 Ž (clearly not steric
in origin) was ascribed to sp hybridization at oxygen with one pair of electrons involved
in -bonding to the phenyl group while the other was donated to the metal centre. As
the database of structurally characterized transition metal aryloxides grew it became
possible to analyse structural parameters to try and detect any correlation between
M–O–Ar angles and the degree of -donation as measured by the M–OAr distance. For
the group 4 and 5 metals Ti, Zr, V, Nb, and Ta studies led to the conclusion that the size
of the M–O–Ar angle was a very poor measure of the degree of -donation. 105,192–194
Plots of M–O distance versus M–O–Ar angle highlight this phenomenon (Fig. 6.1).
Included on the plots are unrestrained terminal aryloxides as well as terminal aryloxides
that are part of a chelate ring. However, in the case of vanadium the large numbers of
Ti−O distance (Å)
2
1.95
1.9
1.85
1.8
1.75
1.7
100 110 120 130 140 150 160 170 180
Ti−O−Ar angle (°)
(a)
Figure 6.1 The variation of metal–OAr distance (as a measure of -donation) with
M–O–Ar angle for (a) titanium, (b) zirconium, (c) vanadium, (d) niobium, (e) tantalum.
ž D four-coordinate, � D five-coordinate, � D six-coordinate, � D sevencoordinate
metal.