2 Homometallic Alkoxides

Metal Aryloxides 467
Two other important binding modes for aryloxides involve metal interactions with the
phenoxide ring. In one situation the aryloxide ligand binds to one metal centre through
the oxygen atom while the phenoxide ring is -bound to another metal centre. 165–167
This type of bonding situation is often encountered for aryloxide derivatives of electropositive
metals where there is also a lack of Lewis bases to provide needed electron
density to the metal centres. The second situation involves the -binding of phenols
or phenoxides to later transition metal centres. 168–171 In some cases this can lead to
5 -bound cyclohexadienonyl ligands. 102,172–174
4.2 Bonding for Terminal Metal Aryloxides
One of the most widely discussed aspects of the bonding of alkoxide, aryloxide, and
related oxygen donor ligands focusses on the presence and extent of oxygen-p to metal
-bonding as well as the possible importance of an ionic bonding model. A simplistic
analysis would conclude that changes in the oxygen atom hybridization from sp 3 to sp 2
and sp allows the oxygen atom to interact with one, two, or three orbitals on the metal
centre. Filling these orbitals with 2, 4, or 6 electrons would lead to formal M–O single
( 2 ), double ( 2 , 2 ) and triple ( 2 , 4 ) bonds respectively. The -components of the
multiple bonds can therefore be thought of as arising from rehybridization of oxygen
lone pair electron density so that it can be donated to the metal centre. In organic chemistry
the -donation of oxygen electron density from hydroxy and alkoxy substituents
is routinely used to rationalize both structure and reactivity. Particularly informative
in this regard are aryl alcohols (phenols) themselves as well as related aryl ethers. The
increased acidity of phenol over simple alkyl alcohols as well as the relative ease of
electrophilic attack on the phenoxide nucleus reflect the delocalization ( -donation)
of oxygen electron density into the aromatic ring. This argument can also be used to
account for the structural parameters of aryl ethers. The O–Ar bond is consistently
shorter than is found for alkyl ethers and the R–O–Ar angle is close to 120 Ž (sp 2 )
with the alkoxy group lying within the arene plane (maximum -orbital overlap). 175
In considering metal aryloxide bonding it is important, therefore, to recognize that the
oxygen atom is bonded to two potential -acceptor groups, the phenoxy ring as well
as the metal centre. A variety of structural and other studies indicate that aryloxides
are weaker -donor ligands than simple alkoxides (see below). 176
M O
Ar
M O
Ar
M O Ar
The existence of aryloxide oxygen to metal -bonding should be manifested in a
variety of ways. The extent of -donation would depend on a large number of interrelated
factors (formal metal oxidation state, molecular symmetry, coordination number,
nature of ancillary ligands, etc.), which ultimately control the electron deficiency of
the metal centre and the availability of suitable empty -acceptor orbitals. How some
of these factors influence various parameters is discussed below.