Chemistry Reference
In-Depth Information
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
O
,
C
can be defined as
O
,
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
O
,
C
of 0
.
15
˚
A(Sn)and0
.
17 A (Ge) implying little or no
-bonding.
105
In
contrast, values of
O
,
C
for identical (to ensure constant ligand electronic and steric
factors) derivatives of Ti and Zr were found to be 0
.
28
˚
Aand0
.
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-OandM-C distances for some of these compounds are presented in Table 6.1
along with calculated values of
O
,
C
for these ligands bound to the same metal centre.
It can be seen that the values of
O
,
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-CH
3
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
O
,
C
D0
.
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
-[NbCl
4
(OC
6
H
3
Me
2
-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
