Chemistry Reference
In-Depth Information
5.5.3
Tantalum
The hexanuclear tantalum heterometallic oxo-isopropoxide [Ta
4
Zn
2
3
-O
2
-O
2
-
OPr
i
6
OPr
i
8
I
2
] has a centrosymmetric structure comprising two triangular
Ta
2
Zn
3
-O
OPr
i
3
I units linked by two nearly linear Ta
-O
Ta bridges. Tantalum
has distorted octahedral and zinc tetrahedral coordination.
23
5.6 Chromium Sub-group Metal-containing Oxo-alkoxides (Including
Heterometallic Compounds) (Table 5.6)
5.6.1
Chromium
A small number of oxo-alkoxides of chromium have been structurally
characterized. The mononuclear chromium(
VI
) compounds [Cr
O
2
Cl
OCPh
3
]and
[Cr
O
2
OCPh
3
2
] both involve tetrahedrally coordinated chromium.
147
Chromium(
V
)
is
five-coordinated
(tetragonal
pyramidal)
with
the
terminal
oxo-ligand
in
[Cr
O
fO
2
C
2
CF
3
4
g
2
]
the
apical
position
in
the
mononuclear
and
binuclear
[Cr
2
O
2
-O
2
fO
2
C
2
CF
3
4
g
2
]
2
anions.
48
5.6.2
Molybdenum
There is an abundance of structurally characterized molybdenum oxo-alkoxides.
Several mononuclear species have been reported. In the
cis
-dioxo molybdenum(
VI
)
derivatives [Mo
O
2
OC
2
H
4
OH
2
]
50
and [Mo
O
2
OCHMeCHMeOH
2
]
148
the metal
is in a distorted octahedral configuration. A similar structure was found in
[Mo
O
2
OC
2
H
4
OMe
2
].
149
The tridentate alkoxo ether ligand OC
2
H
4
OC
2
H
4
Oforms
a
mer
-complex [Mo
O
2
OC
2
H
4
OC
2
H
4
O
] in which Mo achieves six-coordination
by intermolecular bonding (Mo
D
O
....
Mo) giving a linear polymer.
51
A similar situ-
ation arises in the pyridine-2,6-dimethoxo complex [Mo
O
2
f
OCH
2
2
C
5
H
3
Ng].
70
In the octahedral
cis
-dioxo complex [Mo
O
2
OPr
i
2
bpy
] the isopropoxo ligands
are
trans
.
34
The monoxo molybdenum tetrakis-perfluoro-
tert
-butoxo compound
[MoOfOC
CF
3
3
g
4
] exhibits five-coordinated Mo (distorted TBP).
150
Alkoxo-
bridging leads to binuclear species as in all-
cis
-[Mo
2
O
4
-OMe
2
OC
2
Me
4
OH
2
]
151
and [Mo
2
O
2
-OMe
2
OMe
6
].
15
The novel unsymmetrical binuclear species
[
PMe
3
3
Bu
t
CH
2
O
Mo
-O
-OCH
2
Bu
t
Mo
OCH
2
Bu
t
2
PMe
3
] (Mo-Mo, 2.493 A)
contains octahedral and five-coordinated (TBP) Mo atoms, which are edge-
bridged by the oxo and one alkoxo ligand.
152
In the binuclear Mo(
V
) complex
[Mo
2
O
2
-OEt
2
-EtOH
Cl
4
] (Mo-Mo, 2.683 A) there is a confacial bi-octahedral
structure with the rather unusual bridging ethanol molecule.
27
The trinuclear molecules [Mo
3
-O
3
-OR
-OR
3
OR
6
]
R D Pr
i
,CH
2
Bu
t
)
have the familiar triangulo-Mo
3
unit capped above and below the metal plane by
3
-oxo
and
3
-alkoxo ligands. The Mo
3
O
11
framework enables each Mo(
IV
) atom to achieve
a distorted octahedral coordination.
34
The electronic structure of these compounds
153
and their relationship to metal oxide structures
154
have been discussed. Two
trinuclear anions [Mo
3
O
7
hmmp
2
]
2
and [Mo
3
O
6
OMe
hmmp
2
]
(hmmp D
OCH
2
3
CMe) have also been determined.
59
In the former complex two metals are
cis
-bonded to two terminal oxo ligands and the third is facially bonded to three. In the
