Chemistry Reference
In-Depth Information
1,2-Mo
2
OBu
t
2
CH
2
SiMe
3
4
is ethane-like
C
2h
symmetry).
235
Addition of dialkyl
cyanamides R
2
NCN to Mo
2
OR
6
compounds produced [Mo
2
OR
6
-NCNR
2
]com-
plexes and the crystal structure [Mo
2
OR
6
-OCH
2
Bu
t
-NCNMe
2
OCH
2
Bu
t
5
]
was determined. One Mo atom is bonded to two terminal Bu
t
CH
2
O ligands, the
2
-CN
portion of the cyanamide and one bridging Bu
t
CH
2
O ligand. The second Mo is bonded
to three terminal Bu
t
CH
2
O ligands, the bridging nitrogen of the cyanamide and the
bridging Bu
t
CH
2
O ligand. The terminal Bu
t
CH
2
O
trans
to the bridging cyanamide
nitrogen has a longer Mo-O bond length (1
.
954 A) than the other terminal neopen-
toxide groups (av. 1
.
87 A) whilst the alkoxo bridge is unsymmetrical (Mo-O, 1.999,
2
.
146 A).
236
In the dimer [Mo
2
-
1
,
1
-O
2
CPh
2
OBu
t
4
]theMo Mo unit (d, 2
.
236 A) is
spanned by a pair of
cis
bridging benzoato ligands with terminal Bu
t
O ligands
trans
to each of the benzoate oxygens.
237
By contrast in the acetylacetonate
[Mo
2
OBu
t
4
acac
2
](Mo
Mo, 2
.
237 A) the diketonato groups are chelating
in preference to bridging.
238
Addition of
tert
-butyl isonitrile gave the edge-
bridged bi-octahedral molecule [Mo
2
-OCH
2
Bu
t
2
OCH
2
Bu
t
2
CNBu
t
2
acac
2
].
239
Alkoxo-ligands in Mo
2
OR
6
can be replaced by thiolato groups as in
[Mo
2
OPr
i
2
SAr
4
](ArD 2,4,6-trimethylphenyl) where the MM bond distance
(2
.
230 A) is typical for this class of compound.
240
The arylselenato complex
[Mo
2
OPr
i
2
SeAr
4
](Mo
Mo, 2
.
219 A) is similar.
299
In the novel bromo complex
[Mo
2
-OCH
2
Bu
t
2
-Br
OCH
2
Bu
t
4
Br(py)] each Mo is in a distorted octahedral
environment
(2
.
534 A)
and
the
Mo-Mo
distance
implies
metal- metal
double
bonding.
241
Reaction of Mo
2
OBu
t
6
with diphenyldiazomethane gave the mononuclear
five-coordinated (TBP) [Mo
OBu
t
4
NNCPh
2
] where the equatorial Mo-O bond
lengths (av. 1
.
89 A) are shorter than the axial Mo-O bond (1
.
944 A). With
Mo
2
OPr
i
6
in the presence of pyridine the binuclear confacial bi-octahedral
complex [Mo
2
-OPr
i
3
NNCPh
2
2
py
] with Mo-Mo D 2
.
66 A indicative of a single
metal - metal bond.
242
One Mo atom is coordinated to an NNCPh
2
ligand, two terminal
Pr
i
O ligands and three bridging Pr
i
O ligands whilst the other Mo is coordinated to one
NNCPh
2
ligand, one terminal Pr
i
O, one pyridine, and three bridging Pr
i
O ligands.
The bridges are distinctly unsymmetrical. The binuclear phenylisocyanate insertion
product [Mo
2
OPr
i
4
fNPhC
O
OPr
i
g
2
]hastwo
cis
bridging bidentate ligands and
four terminal isopropoxo ligands with a Mo
Mo (2
.
221 A).
243
The quadruply metal- metal bonded dimers [Mo
2
OPr
i
4
py
4
], [Mo
2
OCH
2
Bu
t
4
PMe
3
4
], [Mo
2
OCH
2
Bu
t
4
Me
2
NH
4
], and [Mo
2
OPr
i
4
Pr
i
OH
4
]areofspecial
interest (Mo
Mo; 2.195, 2.209, 2.133, 2
.
110 A, respectively) in having no bridging
ligands.
244
Generally in the [Mo
2
OR
4
L
4
] complexes the OR groups are
cis
to the L
ligands and the two halves of the molecule are in the eclipsed conformation (Fig. 4.41).
When L D Pr
i
OH or Me
2
NH there is hydrogen bonding RO
....
HOR(HNMe
2
)across
the metal - metal bond which is thereby shortened. The same situation occurs in
[Mo
2
OC
5
H
9
-
c
4
c
-C
5
H
9
OH
4
] although it was not possible to distinguish
c
-C
5
H
9
O
ligands from
c
-C
5
H
9
OH in this structure.
245
The
(2
.
235 A)
[Mo
2
OPr
i
4
CH
2
Ph
2
PMe
3
]
unsymmetrical
MoMo
complex
ligand.
246
,
247
has
one
molybdenum
bonded
to
benzyl
groups
and
one
PMe
3
With
the
bidentate
phosphine
Me
2
P
2
CH
2
(dmpm)
the
symmetrical
dimer
