2 Homometallic Alkoxides

428 Alkoxo and Aryloxo Derivatives of Metals
A few structures of tetranuclear tungsten oxo-alkoxides have been reported. In
the remarkable molecule [W4⊲ -O⊳2⊲ -OPr i ⊳3⊲OPr i ⊳9] where W has the average
oxidation state of four there is one triple bonded (W W, 2.404 ˚A) ditungsten
W2⊲OPr i ⊳5 unit linked by two -oxo ligands to one edge of a confacial bioctahedral
W2⊲ 3-OPr i ⊳⊲OPr i ⊳ unit (W–W, 2.684 ˚A). With respect to oxygen the
tungstens are three, four, and six-coordinated. 35 Another unusual structure was found
for [W4⊲ -O⊳⊲ -OPr i ⊳2⊲OPr i ⊳7Cl] which has an apical two-coordinated tungsten
W⊲OPr i ⊳Cl capping a triangle of four-coordinated basal metals linked by metal–metal
bonds. The -oxo ligand bridges two of the basal tungsten atoms which are each
linked to the third basal tungsten by -OPr i ligands. The oxygen coordination around
the basal metals is approximately square planar. 72
In the tetranuclear 4-carbido oxo-alkoxides [W4⊲ 4-C⊳⊲ -O⊳⊲ -OR⊳4⊲OR⊳8] (RD
CH2Bu t ,Pr i ) the four metals adopt the “butterfly” configuration. In the neopentyloxide
the -oxo ligand bridges the backbone tungsten atoms whereas in the isopropoxide
it links one backbone W to a wingtip W. 38 A zig-zag array of four W atoms occurs
in [f⊲CO⊳⊲dmpe⊳2W.W⊲O⊳2g2⊲ -O⊳] (dmpe D Me2PC2H4PMe2) in which the exterior
five-coordinated zero valent tungstens are metal–metal bonded (W–W, 2.648 ˚A) to
three-coordinated pentavalent inner metals, which in turn are linked by the -oxo
ligand. The diamagnetism is presumed to arise from spin pairing of oxo-bridged interior
d 1 -d 1 metal centres. 171
The two centrosymmetrical hexanuclear oxo-alkoxides [W6⊲ -O⊳4⊲ -CR⊳2⊲OBu t ⊳10]
(R D Et, Pr n ) reported 164,172 have the same general structure involving two trinuclear
subunits joined by a pair of unsymmetrical -oxo bridges (W–O, 1.755 ˚A, 2.119 ˚A).
Each isosceles W3 triangle has a -oxo and a -carbyne ligand bridging the longer sides
and each metal has two terminal tert-butoxo groups. The only octanuclear tungsten oxoalkoxide
reported to date is the dimeric W4 unit in [W8⊲ -O⊳4⊲ -OPr i ⊳4⊲OPr i ⊳12]. 173
This molecule is centrosymmetric and the two W4 units are connected by a
W⊲ -OPr i ⊳2W double bridge. Each W4 unit consists of a W3 triangle bridged by two -
oxo and one -OPr i group, and linked to the fourth tungsten by three tungsten–tungsten
bonds.
5.7 Manganese Sub-group Metal-containing Oxo-alkoxides (Including
Heterometallic Compounds) (Table 5.7)
5.7.1 Manganese
The use of the tridentate alkoxo ligand MeC⊲CH2O⊳3 gave rise to the decanuclear mixedvalence
manganese oxo-alkoxo-chloro anion [Mn10⊲ 6-O⊳2f⊲OCH2⊳3CMeg6Cl8] 2 . 88
The Mn10O20Cl8 structure (Fig. 5.20) can be considered as the fusion of two octahedral
Mn6O13Cl6 units by eliminating two cis manganese moieties. The six alkoxo
ligands symmetrically cap the eighteen surface bridging oxo-sites and the mixed valence
is accounted for by eight Mn(III) and two Mn(II) atoms.
The remarkable mixed-ligand mixed valence neutral tridecanuclear molecule
[Mn13⊲ 5-O⊳6⊲ 3-O⊳2⊲ 3-OEt⊳6⊲ -O2CPh⊳12] has a supercubane structure with a
central Mn(IV) atom octahedrally surrounded by six 5-oxo ligands which are
symmetrically bonded to six Mn(III) and six Mn(II) atoms whose octahedral coordination
is completed by the twelve bidentate bridging benzoate ligands. 89