2 Homometallic Alkoxides

468 Alkoxo and Aryloxo Derivatives of Metals
4.2.1 Terminal M–OAr Bond Distances
The arguments presented above imply that as the amount of oxygen to metal -donation
increases so the M–O distance should decrease. The question therefore arises as to
what the metal–oxygen distance should be for an aryloxide ligand that is undergoing
no -bonding with the metal centre. Two approaches have been taken to answer this
question. The first method tries to estimate what a metal–aryloxide single bond distance
should be using structural parameters for ligands that cannot themselves -bond to
metal centres. This approach was originally applied to metal dialkylamido ligands by
Chisholm et al. 177 Hence on the basis of known metal–alkyl bonds and the difference
in covalent radii for carbon and oxygen it is possible to estimate what a particular
metal aryloxide bond should be in the absence of -bonding. This approach has been
successfully applied in the literature to metal aryloxide derivatives of both p- and
d-block metals. From covalent radii obtained from organic structures it appears that
(element)E–O(aryloxide, alkoxide) bonds are approximately 0.10–0.15 ˚A shorter than
corresponding element–alkyl bonds. The parameter 1O,C can be defined as
1O,C D d⊲M–O⊳ d⊲M–C⊳
andusedtoestimatetheextentofany -bonding of aryloxide ligands. Applying this
analysis to four-coordinate aryloxide derivatives of the group 14 metals showed values
of 1O,C of 0.15 ˚A (Sn)and 0.17 ˚A (Ge) implying little or no -bonding. 105 In
contrast, values of 1O,C for identical (to ensure constant ligand electronic and steric
factors) derivatives of Ti and Zr were found to be 0.28 ˚A and 0.29 ˚A, showing
the presence of considerable oxygen-p to metal-d -bonding. In the case of the
group 5 metals Nb and in particular Ta there are now many structurally characterized
organometallic compounds of the type [M(OAr)x(R)5 x]. The predominant structural
type is trigonal bipyramidal with a few examples of square-pyramidal geometry. The
M–O andM–C distances for some of these compounds are presented in Table 6.1
along with calculated values of 1O,C for these ligands bound to the same metal centre.
It can be seen that the values of 1O,C for these compounds are lower than the 0.1
to 0.15 ˚A predicted for purely bonding.
In the case of the group 5 metals Nb and Ta and the group 6 metal W there is
a second way to measure the shortening of the M–OAr bond due to -bonding.
For these metals there exist formally saturated (18-electron) derivatives containing
aryloxide ligation. For the group 5 metals the compounds [MeTa(dmpe)2(CO)2] and
[(ArO)Nb(dmpe)2(CO)2] have been structurally characterized. The Nb–OAr bond
length of 2.181 (4) ˚A compares with a Ta–CH3 distance of 2.32 (1) ˚A found for
the alkyl. Given the negligible difference found between the M–L bond lengths for
derivatives of Nb and Ta we can calculate 1O,C D 0.14 ˚A for these two derivatives.
We can now observe how the M–OAr bond length varies as the “formal” electron count
at the metal (i.e. electron count in the absence of -donation) is decreased. Some of this
data is presented in Table 6.2. It can be seen that the M–O distance drops dramatically,
as the metal centre becomes more electron deficient. The shortest distance of 1.819
(8) ˚A is found for the compound trans-[NbCl4(OC6H3Me2-2,6)(THF)] in which the
metal centre is attached to four electronegative chloride ligands which are poorer -
donors than the aryloxide ligand. 178 The decrease of 0.36 ˚A from the distance found
in the di-carbonyl compound is comparable to the decrease of the M–C interatomic