Chemistry Reference
In-Depth Information
Although Mo
2
OBu
t
6
reacts readily with CO to form Mo
CO
6
and Mo
OBu
t
4
(Eq. 4.1),
2Mo
2
OBu
t
6
C 6CO ! Mo
CO
6
C 3Mo
OBu
t
4
4
.
1
the dinuclear carbonyl complex [Mo
2
-OBu
t
2
-CO
OBu
t
4
] was isolated as an
unstable intermediate.
227
The molybdenum atoms are in distorted square pyramidal
five-coordination sharing a common triangular face with the carbonyl carbon atom at
the common apex. This diamagnetic compound appears to involve MoDMo (Mo-Mo,
2
.
498 A).
Addition of nitric oxide to Mo
2
OPr
i
6
gave the diamagnetic dimer [Mo
2
-
OPr
i
2
OPr
i
4
NO
2
] comprising edge-bridged trigonal bipyramids. The near linear
(MoNO, 178
Ž
) nitrosyls occupy terminal axial positions and the two bridging
isopropoxo groups form short bonds in the equatorial positions and long bonds in
the axial positions. The Mo-Mo distance (3
.
335 A) precludes metal - metal bonding.
228
The exceptionally long axial bridging Mo-O bond (2
.
195 A
vs
1
.
951 A equatorial)
reflects the high
trans
influence of the NO ligand. Addition of dimethylamine
to [Mo
2
-OPr
i
2
OPr
i
4
NO
2
] gave the unstable [Mo
2
-OPr
i
2
OPr
i
4
Me
2
NH
2
]
which has the edge-shared isopropoxo-bridged bi-octahedral structure.
229
The linear
NO ligands are
trans
to the unsymmetrical bridging isopropoxo groups which again
reveal the
trans
influence of the nitrosyl groups. The terminal Pr
i
O group
trans
to a
bridging Pr
i
O has a significantly shorter Mo-O bond length (Mo-O, 1
.
909 A) than the
terminal Pr
i
O group
trans
to the Me
2
NH ligand (Mo-O, 1
.
935 A).
Addition of halogen X
2
(X D Cl, Br) to Mo
2
OPr
i
6
gave the unstable halide alkox-
ides [Mo
2
-OPr
i
2
OPr
i
4
X
4
].
230
Each compound has the edge-shared isopropoxo-
bridged bi-octahedral structure with
cis
terminal halides and
trans
terminal Pr
i
O
ligands. The Mo-Mo distances (X D Cl, 2
.
731 A; X D Br, 2
.
739 A) are consistent
with the presence of single metal - metal bonds in these Mo(
V
) complexes. The very
short Mo-O bond distances for the terminal Pr
i
O groups suggest that
-donation for
Pr
i
O oxygen to molybdenum is very pronounced.
Addition of arylazides ArN
3
to Mo
2
OBu
t
6
gave rise to the dinuclear bis-
tert
-
butoxo-bis-arylimido-molybdenum(
VI
) complexes [Mo
2
-NAr
2
(NAr
2
(OBu
t
4
].
231
In the
p
-tolyl derivative the dimer consists of two equatorial - axial edge-sharing
trigonal bipyramids with terminal tolylimido ligands (near linear MoNC) The Bu
t
O
ligands occupy equatorial positions. Addition of diphenyldiazo methane to Mo
2
(OR)
6
in the presence of pyridine gave the dinuclear [Mo
2
-OR
3
OR
3
(N
2
CPh
2
2
(py)](R D
Pr
i
,CH
2
Bu
t
).
232
The unsymmetrical molecule (Fig. 4.39) has a confacial Pr
i
O-bridged
bioctahedral structure with a terminal NNCPh
2
ligand attached to each Mo. One Mo
has two terminal Pr
i
O ligands and the other has one Pr
i
O and a pyridine ligand in
terminal positions. The Mo-Mo distance (2
.
662 A) is consistent with the presence of
a single metal- metal bond.
Addition of alkynes RC
2
R
0
to Mo
2
(OR)
6
(R
00
D Pr
i
,Bu
t
,CH
2
Bu
t
) in the presence
of pyridine gave rise to alkyne-bridged alkoxo-bridged complexes [Mo
2
(
-RC
2
R
0
)(
-
OR)
2
(OR)
4
(py)
2
].
233
The structure of [Mo
2
(
-C
2
H
2
)(
-OPr
i
)
2
(OPr
i
)
4
(py)
2
] revealed
that the acetylene molecule bridges the two molybdenum atoms in a crosswise manner
(Fig. 4.40) with two bridging Pr
i
O groups in a pseudo-confacial bi-octahedron config-
uration and the pyridines
trans
to the
-C
2
H
2
group. The Mo-Mo distance (2
.
554 A)
is suggestive of a metal - metal double bond. In the case of the neopentoxide, addition
