2 Homometallic Alkoxides

466 Alkoxo and Aryloxo Derivatives of Metals
where ArO = O Me.
where ArO = HO .
Cp Ł 2 Th⊲H⊳2 C HOAr ! Cp Ł 2 Th⊲H⊳⊲OAr⊳ C H2 ⊲6.58⊳
Me 3C
Me 3C
4 BONDING OF METAL ARYLOXIDES
4.1 Bonding Modes
Bradley delineated the important factors that affect the solution and solid-state
structures adopted by metal alkoxide compounds many years ago. 156 A similar
situation exists for metal aryloxides. The aryloxide ligand can adopt a variety
of bonding modes. Quadruply (rare) and triply bridging aryloxides are typically
restricted to compounds containing small phenoxides. The commonest examples
are contained in cluster compounds of the group 1 and group 2 metals and
mixed metal derivatives of the lanthanides. For the group 1 metals the archetypal
structures are hexagonal and cubic clusters, e.g. [Li6⊲ 3-OPh)6(THF)6] 157 and [Na4⊲ 3-
OAr)(DME)4] 158 (OAr D OC6H4-4Me). Triply bridging phenoxides are also common
in tri- and hexanuclear clusters of Ca, Ba, and Sr, e.g. [Ca3(OPh)5(HMPA)6], 159
[Ba6(OPh)12(TMEDA)4] 159 and [Sr3(OPh)6(HMPA)5] 159 . Other examples containing
triply bridging aryloxides are mixed metal clusters such as [(py)4Na4Cr2(OPh)8] 160
and [(THF)4Na4Cr2(OPh)8]. 161 A few mixed metal clusters of the lanthanides have
been shown to contain quadruply bridging aryloxides, e.g. [Me4N][La2Na2( 4-
OAr)( 3-OAr)2( 2-OAr)4(OAr)2(THF)5] (OAr D 4-methylphenoxide). 162 As expected
the M–OAr distance increases progressively on moving from terminal to doubly
and triply bridging bonding mode. This is highlighted by the compound
[Ba5(OPh)9(H)(O)(THF)8] 163 where the Ba–OAr distances are 2.54 (1) ˚A (terminal),
2.65 (3) ˚A (av., 2) and 2.75 (3) ˚A (av., 3). 163
A large number of compounds contain doubly bridging aryloxides. Some of these
compounds contain aryloxides bridging between identical metals, but a large number
involve M–O–M 0 bridges where one of the metals is a group 1 element. In the majority
of cases involving identical elements the two M–O distances and M–O–Ar angles are
very similar. There are, however, some examples in which there is a pronounced
asymmetry to the aryloxide bridge. As an example, in the compound [Li3( 2-OAr)3]
(OAr D OC6HBu t 2 -3,5-Ph2-2,6) which contains two coordinate lithium atoms the Li–O
distances are 1.78 (1) ˚A and 1.840 (7) ˚A while the Li–O–Ar angles are 113.8 ⊲3⊳ Ž and
134.9 ⊲3⊳ Ž . 164
By far the commonest bonding mode for aryloxide ligands is a terminal one in
which the phenoxide oxygen atom is bound to only one metal atom. In some cases
the phenoxide function may also be a part of a chelate ring and this may constrain
the M–O–Ar angle (see below). Chelation may occur via donor heteroatoms or via
metallated or -bound organic substituents (Section 2.1.2).