Chemistry Reference
In-Depth Information
isopropanol in [Hf
2
OPr
i
8
Pr
i
OH
2
] may be replaced by pyridine and the structure
of [Hf
2
-OPr
i
2
OPr
i
6
Pr
i
OH
py
] shows that the edge-shared bi-octahedral
configuration is retained. The coordinated Pr
i
OH molecule is clearly hydrogen bonded
to the terminal isopropoxo group on the adjacent hafnium atom.
200
In the mixed ligand
complex [
Bu
t
O
3
Hf
-Cl
-OBu
t
-NHPMe
3
Hf
OBu
t
3
] the structure comprises
a confacial bi-octahedral configuration with three different bridging ligands.
198
A
mononuclear octahedral hafnium compound [Hf
OSiPh
2
OSiPh
2
OSiPh
2
O
2
py
2
]was
obtained using the chelating hexaphenyl trisiloxane diolate ligands supplemented by
pyridine donors in the
cis
configuration. Eight-membered hafnasiloxane rings are
present in a distorted octahedral structure.
199
The Hf-O bonds
trans
to the pyridines
are shorter (1
.
961 A) than those
cis
(2
.
014 A).
2.10
Alkoxides of Vanadium, Niobium, and Tantalum
2.10.1
Vanadium
The structures of many alkoxo vanadium compounds have been reported but they
all contain oxo ligands and are accordingly dealt with in the chapter on metal oxo
alkoxides.
2.10.2
Niobium
The Nb(
V
) complex [Nb
OPr
i
3
diket
(NCS)] (diket D PhCO
CH
COPh) has
a
fac
-octahedral structure.
202
The first homoleptic niobium alkoxide structure
determined was the dimeric pentamethoxide [Nb
2
-OMe
2
OMe
8
]
203
and this
confirmed the edge-shared bi-octahedral configuration (Fig. 4.2) originally proposed
on the basis of variable-temperature
1
H NMR spectroscopy.
204
Two independent
centrosymmetrical molecules are present in the unit cell (data in Table 4.10, p. 281).
The niobium(
IV
) dichloride dimethoxide complexes [Nb
2
-OMe
2
OMe
2
Cl
4
L
2
]
(L D MeOH, MeCN) also exhibit the edge-shared bi-octahedral structure.
205
In the
methanolate the four chlorines occupy the same plane as the Nb
-OMe
2
Nb double
bridge. Interestingly the methanol hydroxyls form intermolecular hydrogen bonds to
neighbouring chlorides. In the acetonitrile complex the MeCN ligands are in the
ClNb (
-OMe
2
NbCl plane with methoxo groups
trans
to chlorines in the axial
positions. These diamagnetic molecules have d
1
- d
1
,Nb-Nb single bonds (Nb-Nb,
2
.
781 A, 2
.
768 A) in contrast to the d
0
[Nb
2
OMe
10
](Nb-Nb, 3
.
5 A). A similar
structure is shown by the Nb(
III
) complex [Nb
2
-OPr
i
-Cl
Pr
i
OH
4
Cl
4
] which has
aNb
-OPr
i
-Cl
Nb double bridge.
206
The Pr
i
OH ligands
trans
to chlorine (Nb-O,
2
.
09 A, 2
.
13 A) are significantly shorter than those
trans
to the bridging isopropoxo
group (Nb-O, 2
.
22 A, 2
.
27 A) and there is intramolecular H
....
Cl hydrogen bonding.
The formal double bond (Nb
D
Nb) character is reflected in the short niobium - niobium
distance (2
.
611 A).
The [Nb
2
-OMe
3
OMe
6
]
anion has the confacial bi-octahedral structure in three
related salts [Mg
MeOH
6
]
2
[Nb
2
OMe
9
]Cl
3
,[Mg
MeOH
6
][Nb
2
OMe
9
]I.2MeOH
and [Na
MeOH
6
][Nb
2
OMe
9
].
207
The niobium - niobium bond distance is quite short
single bond (Nb-Nb, 2.632 - 2.652, av. 2
.
640 A) whilst the
for a formally d
1
- d
1
