Chemistry Reference
In-Depth Information
terminal Nb-OMe bonds are longer than normal owing to interionic hydrogen bonding
with the methanol molecules of neighbouring complex cations.
The steric effect of the bulky N
SiMe
3
2
ligand is evident in the structure of
[Nb
2
-OMe
2
Cl
2
NSiMe
3
2
fN
SiMe
3
2
g
2
] which is constrained to five-coordination
for the Nb(
V
)atoms.
208
2.10.3
Tantalum
The organotantalum alkoxo compound [Ta
OBu
t
2
CHBu
t
f
Me
2
N
2
C
6
H
3
g] s
mononuclear with a distorted trigonal pyramidal configuration.
209
In the
seven-coordinated cation [TafO
C
2
H
4
O
3
C
2
H
4
OgCl
2
]
2C
all five oxygens of the
tetraethyleneglycolate are in the pentagonal plane.
210
In the mixed ligand complex
[Ta
2
-OMe
2
OMe
2
NSiMe
3
2
fN
SiMe
3
2
g
2
] the binuclear five-coordinated Ta(
V
)
molecule involves methoxo bridges.
254
2.11
Alkoxides of Chromium, Molybdenum, and Tungsten
2.11.1
Chromium
The structures of a few alkoxo compounds of chromium have been determined.
The first homoleptic molecule to be structurally characterized was the monomeric
Cr(
IV
) tetrakis-(2,2,4,4-tetramethyl-3-pentanolate) [Cr
OCHBu
t
2
4
] which exhibited
a slightly distorted tetrahedral configuration
211
(data in Table 4.11, pp. 300 - 319).
With the functionalized alkoxo group OCMe
2
CH
2
OMe the tris-chelated compound
[Cr
OCMe
2
CH
2
OMe
3
] was characterized as the
mer
octahedral Cr(
III
) complex.
212
A novel trigonal planar three-coordinated Cr(
II
) complex was obtained as the
dimer [Cr
2
-OCHBu
t
2
2
OCBu
t
3
2
] using bulky alkoxo ligands.
213
The terminal
Cr-O-C bonds were almost linear and the short Cr-O bond lengths suggested
that ligand-to-metal
-bonding was operative. A dimeric structure was also found
in the cyclopentadienyl chromium-
tert
-butoxide [
C
5
H
5
2
Cr
2
-OBu
t
2
].
214
Magnetic
susceptibility measurements revealed antiferromagnetism in this Cr(
II
)(d
4
) dimer and
the short Cr-Cr distance (2
.
65 A) suggests that metal- metal bonding is present.
2.11.2
Molybdenum
The first reported structure of a mononuclear alkoxo molybdenum compound
featured the octahedral Mo(
IV
) trialkylsiloxy complex [
trans
-Mo
OSiMe
3
4
Me
2
NH
2
]
which had nearly linear Mo-O-Si bond angles (Table 4.11).
215
The remark-
able carbonyl complex [Mo
OBu
t
2
CO
2
py
2
], which is formally Mo(
II
), has
trans-tert
-butoxo ligands with
cis
carbonyls and
cis
pyridines.
216
The bulky 1-
adamantolato ligand gave a mononuclear five-coordinated (distorted TBP) complex
[Mo
1-ado
4
Me
2
NH
](1-adoD 1
adamantolato)
with
the
dimethylamine
ligand
occupying an axial position.
217
The difference in bond lengths (0
.
066 A) between
axial Mo-O(1
.
963 A) and the average equatorial Mo-O(1
.
897 A)
the
is note-
worthy. See also [Mo
OBu
t
4
NNCPh
2
].
305
The tridentate 3,5-dimethylpyrazolylato
[Mo(OR)
OR
0
(NO)
Me
2
pz
]
ligand
forms
octahedral
nitrosyl
alkoxo
complexes
