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metals bonded to terminal oxo ligands
trans
to the
5
-oxo ligand. The four
-OMe
ligands bridge the apical Mo to the basal metal atoms which are linked to each other
by
-oxo ligands. The basal Mo atoms also have terminal oxo ligands (
trans
to the
bridging
-OMe oxygens) which are linked to the sodium atom. Each Mo is thus in a
distorted octahedral configuration.
5.6.3
Tungsten
Like molybdenum its congener tungsten is represented by many oxo-alkoxides
with various nuclearities. Among the mononuclear species the W(
VI
) anion
[W
O
2
OCMe
2
CMe
2
O
CH
2
Bu
t
]
exhibits trigonal pyramidal five-coordination
with the terminal oxo ligands occupying equatorial positions with the neopentyl
group in an axial site.
71
Two neutral octahedral complexes [W
O
OBu
t
4
thf
]
164
and [W
O
OSiMe
2
Bu
t
4
py
]
41
have been reported. In the mixed ligand neutral oxo-
alkoxide-aryloxide [W
O
OBu
t
2
OC
10
H
5
Me
.
C
10
H
5
MeO
] the oxo ligand occupies
an equatorial site in a distorted TBP structure.
165
The binuclear W(
VI
) complex [W
2
O
2
-O
2
OSiMe
2
Bu
t
4
py
2
]hasa
-O
2
edge-shared bioctahedral structure with the terminal oxo-ligands
trans
to the pyridines
whereas the W(
IV
) compound [W
2
-O
-OBu
t
OSiMe
2
Bu
t
5
py
2
] (W-W,
2.488 A) has an unsymmetrical structure with octahedral and TBP metals bridged
by
-oxo and
-
tert
-butoxo ligands.
41
By contrast the W(
VI
) phenylimido complex
[
PhN
Bu
t
O
2
W
-O
-OBu
t
2
W
NPh
OBu
t
2
] has a confacial bi-octahedral
structure with a
-oxo and two
-OBu
t
bridges
40
and the phenylimido diolato complex
[
PhN
OC
2
Me
4
O
HOC
2
Me
4
O
W
-O
W
NPh
OC
2
Me
4
O
OC
2
Me
4
OH
]hasan
apex sharing bioctahedral structure.
166
The dimeric molecules [W
2
O
2
-OR
2
OR
6
](RD Me,
c
-C
6
H
11
)haveterminal
oxo ligands
trans
to the bridging alkoxo groups in an edge-shared bioctahedral
structure.
49
An unsymmetrical binuclear W(
V
) molecule was found in [
CF
3
CO
2
2
Bu
t
CH
2
O
2
W
-
OCH
2
Bu
t
-CMe
2
W
O
OCH
2
Bu
t
py
] which had one octahedral and one TBP
metal.
36
In the related molecule [
Bu
t
CH
2
O
3
W
-OCH
2
Bu
t
2
-CHC
3
H
5
-
c
-
W
O
OCH
2
Bu
t
py
] (W-W, 2.659 A) both metals achieve octahedral coordination
through the triple-bridged confacial bi-octahedral structure.
37
More examples of
-oxo apex-sharing bi-octahedral structures are given by the diolato
complexes [
HOC
2
Me
4
O
OC
2
Me
4
O
O
W
-O
W
O
OC
2
Me
4
O
OC
2
Me
4
OH
],
[
HOC
7
H
12
O
O
2
C
7
H
12
-
c
O
W
-O
W
O
O
2
C
7
H
12
-
c
OC
7
H
12
OH
], and
[
HOC
8
H
14
O
O
2
C
8
H
14
-
c
O
W
-O
W
O
O
2
C
8
H
14
-
c
OC
8
H
14
OH
].
73
The novel
W(
IV
) binuclear anion [
Bu
t
O
3
W
-O
-H
W
OBu
t
3
]
(W-W, 2.445 A) contains
five-coordinated tungsten (TBP).
26
The structure of [W
3
3
-O
3
-OPr
i
-OPr
i
3
OPr
i
6
] has been reported
168
and
it is isomorphous with [Mo
3
3
-O
3
-OPr
i
-OPr
i
3
OPr
i
6
].
34
The trinuclear
compounds [W
3
3
-O
3
-OPr
i
-OPr
i
3
OPr
i
4
R
2
](RD CH
2
Ph, Ph) have related
structures.
169
A variation on these trinuclear structures was found in the W(
IV
) complex
[W
3
-O
2
OBu
t
8
] which has an isosceles triangle of metal atoms (W-W, 2.45 A,
2.93 A) with
tert
-butoxo bridges across the two longer sides and no capping ligand.
Each tungsten atom has a different coordination with respect to oxygen (three, four
and five).
170
