2 Homometallic Alkoxides

Metal Aryloxides 477
as expected are for more electronically saturated compounds. It is difficult, however,
to discern any strong trends in these plots. This is particularly true for the group 5
metals Nb and Ta where bond distances of 1.9 ˚A are associated with angles ranging
from 135 Ž to 180 Ž . Interestingly in the case of niobium the longest Nb–OAr distances
are for the seven-coordinate compounds [Nb(OC6H4Me-4)(dmpe)2(CO)2] (18-electron)
and [Nb(O)(quinol)3] with distances [angles] of 2.181 (4) ˚A [160 Ž ] and 2.055 (av.) ˚A
[125 Ž ] respectively. In the latter compound the Nb–O–Ar angle is constrained to 125 Ž
(see Section 6.1.1).
Moving across the d-block one predicts that -bonding effects should decrease except
for the highest oxidation state metal centres. In Tables 6.30–6.47 are collected parameters
for structurally characterized, non-chelated terminal aryloxide compounds of the
group 7–12 metals. These tables show that apart from a few exceptions the M–O–Ar
angles lie in the 120–140 Ž range. Notable exceptions are Mn and Fe derivatives of the
bulky ligands containing 2,6-di-tert-butyl or 2,6-di-(trifluoromethyl) substituents where
almost linear angles are observed.
This situation contrasts with that reported for metal alkoxides where there appears to
be a reasonable correlation between M–OR distances and M–O–R angles. 195 Furthermore,
it has been shown in the compound 1,2-[Mo2(OC6H3Me2-2,6)4(OPr i )2] that
although the M–O(alkoxide) distances are shorter than the M–O(aryloxide) distances,
the M–O–Ar angles exceed the M–O–R angles. 196 The consistently large angles found
for metal aryloxides may be ascribed to the attachment to the oxygen of two -
accepting substituents which can compete for its electron density. In the case of highly
electropositive metal centres, the presence of significant ionic bonding may also be
contributing to the high flexibility of the M–O–Ar angle. 183,189
4.2.3 Electrochemical and Spectrochemical Studies
It is one of the basic tenets of inorganic/organometallic chemistry that -acceptor
ligands such as carbon monoxide stabilize electron-rich metal centres, i.e. metals in
their lowest formal oxidation states. Conversely it is to be expected that a preponderance
of -donor ligands should lead to a stabilization of high valent metal centres. The
-bonding analysis discussed at length above leads to the prediction that as -donor
ligands are added to a mononuclear d 0 -metal centre any empty metal-based orbitals will
be pushed up in energy as they take on Ł character. Hence population of these orbitals,
i.e. reduction of the metal centre, should be made more difficult. This hypothesis was
first interrogated by studying the one-electron reduction potentials for the titanium
aryloxides [Ti(OAr)x(X)4 x] (x D 2, 3, 4) where X is a halide and the aryloxide ligand
is bulky enough to maintain mononuclearity. It was shown by electrochemical studies
(cyclic voltammetry) that as an aryloxide group replaced a chloride ligand the metal
centre became more difficult to reduce by ¾400 mV. 197 Much more extensive studies
have been carried out on aryloxide derivatives of tungsten. 198–202 Work has focussed
on the series of compounds [W(OAr)xCl6 x] and [W(O)(OAr)xCl4 x]. In both cases
it was shown that as a chloride was replaced by an aryloxide the reduction potential
forformationofthed 1 -anion became more negative by 200–300 mV. It was furthermore
shown that the reduction potential could be “tuned” by the choice of substituents
attached to the phenoxide nucleus, electron-donating substituents making it even more
difficult to add an electron to the metal centre.