2 Homometallic Alkoxides

X-Ray Crystal Structures of Alkoxo Metal Compounds 247
tetrahedral tin. 88 The tetraisopropoxide crystallized with coordinated isopropanol giving
the dimeric [Sn2⊲ -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 structure
was found for the isobutoxo complex [Sn2⊲ -OBu i ⊳2⊲OBu i ⊳6⊲Bu i OH⊳2] (data in
Table 4.4). 89 In [Sn2⊲ -OCH2Ph⊳2⊲OCH2Ph⊳6⊲Me2NH⊳2] the terminal alcohol ligand
was replaced by dimethylamine but the bi-octahedral structure was the same. 90 The
tetrakis-2-phenylpropoxide Sn(OCMe 2Ph⊳4 on thermolysis (110 Ž C) undergoes cyclometallation
to form the dimeric [Sn2⊲ -OCMe2C6H4⊳2⊲OCMe2C6H4⊳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 [Sn2⊲ -
OBu i ⊳2⊲OBu i ⊳2⊲Bu i OH⊳2Cl2] has the familiar bi-octahedral structure 91 whereas the
triphenylsiloxo complex with pyridine [Sn(OSiPh 3⊳4⊲C5H5N⊳2] is a mononuclear octahedral
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 2methoxyethoxo
ligand [Pb( -OC2H4OMe⊳2]n which forms chains of apical–equatorial
edge-shared trigonal bipyramids with one equatorial position occupied by the stereochemically
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 ⊳3Pb( -OBu t ⊳3Pb] 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 ⊳3M 0 ⊲ -OBu t ⊳3M] 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 [Sb2⊲ -OMe⊳2(OMe) 8] with
the familiar edge-sharing bi-octahedral structure (Fig. 4.2). 95 The p-tolyl antimony-bistetramethylethyleneglycolate
forms a mononuclear five-coordinated Sb(V) com-
Figure 4.17 Chain structure of [Pb( -
OC2H4OMe)2]n (C and H atoms omitted).