Chemistry Reference
In-Depth Information
tetrahedral tin.
88
The tetraisopropoxide crystallized with coordinated isopropanol giving
the dimeric [Sn
2
-OPr
i
2
OPr
i
6
Pr
i
OH
2
] species involving edge-sharing octahedra
(
cf.
Fig. 4.2) distorted by the presence of asymmetric hydrogen bonding between axially
coordinated alcohol ligands and isopropoxo groups.
88
Thus the terminal isopropoxides
trans
to the bridging ligands have significantly shorter Sn-O bonds (1.943 A) than those
involved in hydrogen bonding (2.098 A and 2.142 A) which are longer than the bridging
ligands (2.085 A). Although the OH-proton was not located, it is probably located
nearer to the isopropoxo group with the longest Sn-O bond (2.142 A). A similar struc-
ture was found for the isobutoxo complex [Sn
2
-OBu
i
2
OBu
i
6
Bu
i
OH
2
] (data in
Table 4.4).
89
In [Sn
2
-OCH
2
Ph
2
OCH
2
Ph
6
Me
2
NH
2
] the terminal alcohol ligand
was replaced by dimethylamine but the bi-octahedral structure was the same.
90
The
tetrakis-2-phenylpropoxide Sn(OCMe
2
Ph
4
on thermolysis (110
Ž
C) undergoes cyclo-
metallation to form the dimeric [Sn
2
-OCMe
2
C
6
H
4
2
OCMe
2
C
6
H
4
2
] containing both
bridging and chelating ligands with five-coordinated (distorted TBP) tin atoms with
the Sn-C(aryl) bonds occupying equatorial positions
90
(data in Table 4.4). Other
octahedral tin alkoxo compounds have been reported. The dimer [Sn
2
-
OBu
i
2
OBu
i
2
Bu
i
OH
2
Cl
2
] has the familiar bi-octahedral structure
91
whereas the
triphenylsiloxo complex with pyridine [Sn(OSiPh
3
4
C
5
H
5
N
2
] is a mononuclear octa-
hedral complex
92
with the pyridine ligands occupying
trans
positions.
2.4.3
Lead
The first lead alkoxide to be structurally determined was the Pb(
II
) derivative of the 2-
methoxyethoxo ligand [Pb(
-OC
2
H
4
OMe
2
]
n
which forms chains of apical - equatorial
edge-shared trigonal bipyramids with one equatorial position occupied by the stereo-
chemically active lone pair of electrons. The ligands are monodentate and form
unsymmetrical alkoxo-bridges (Fig. 4.17).
93
With the bulky
tert
-butoxo lligand the
trinuclear compound [Pb(
-OBu
t
3
Pb(
-OBu
t
3
Pb] was obtained.
94
The molecule has
a linear system of three lead atoms with the central one being octahedrally coordinated
by the six bridging
tert
-butoxo groups and the outer Pb atoms being three-coordinated
(data in Table 4.4). This molecule appears to be the prototype for a large number of
heterotrinuclear species [M(
-OBu
t
3
M
0
-OBu
t
3
M] where M D Ge and M
0
D Mg,
Ca, Cd, Eu, Pb; M D Sn and M
0
D Ca, Sr, Cd, Pb; and M D Pb and M
0
D Sr, Ba.
94
2.5
Alkoxides of p-Block Elements; Sb, Bi
Antimony pentamethoxide forms a dinuclear molecule [Sb
2
-OMe
2
(OMe)
8
] with
the familiar edge-sharing bi-octahedral structure (Fig. 4.2).
95
The
p
-tolyl antimony-bis-
tetramethylethyleneglycolate
forms
a
mononuclear
five-coordinated
Sb(
V
)com-
Figure 4.17
Chain structure of [Pb(
-
OC
2
H
4
OMe)
2
]
n
(C and H atoms omitted).
