2 Homometallic Alkoxides

250 Alkoxo and Aryloxo Derivatives of Metals
pound with the TBP configuration. Each chelating glycolate ligand bonds at one
axial and one equatorial position and the p-tolyl group occupies the remaining equatorial
position. 96 Antimony trimethoxide forms a layer structure [Sb⊲ -OMe⊳3]n in
which each Sb(III) is linked by unsymmetrical methoxo bridges to three others giving
a coordination number of 6. 97 The first bismuth alkoxide structure was determined
simultaneously by two independent groups. 98 The compound [Bi2⊲ -
OC2H4OMe⊳4⊳4⊲OC2H4OMe⊳2]n has a chain structure comprising dimeric units involving
five-coordinated (square pyramidal) bismuth atoms bonded to one terminal (apical)
and four bridging (shared with two neighbours) 2-methoxyethoxo ligands. The edgesharing
square pyramids have unsymmetrical alkoxo bridges and it is possible that the
vacant octahedral site on the Bi atom is occupied by the lone electron pair. None of
the ligands is chelating (Fig. 4.18).
The monomeric mixed ligand complex [Bi(OSiPh 3⊳3(thf) 3] has a distorted octahedral
configuration owing to the presence of the bismuth lone pair and the two sets
of ligands are arranged meridionally. 99 In the dimeric [Bi⊲ -hfip) 2 (hfip) 4 (thf) 2](hfip D
OCH⊲CF3⊳2) the bismuth adopts the square pyramidal five-coordination with lone pairs
occupying the vacant octahedral sites. The edge-sharing bridges are markedly unsymmetrical
with the THF ligands bonded trans to the shorter Bi–O bridge bond (Fig. 4.19). 100
Similar structures were reported for the neopentyloxides [Bi2⊲ -ONp⊳2⊲ONp 4 ⊲NpOH⊳2]
⊲Np D CH2Bu t 3 ⊳ and [Bi2⊲ -ONp⊳4⊲C5H5N⊳2] (data in Table 4.5). 101
2.6 Alkoxides of Scandium and Yttrium
2.6.1 Scandium
Owing to its relative scarcity and lack of any rich mineral sources the chemistry of
scandium had been neglected until recently.
The first crystal structure determined of a scandium alkoxide was that of the dimeric
tert-butoxide [Sc2⊲ -OBu t ⊳2⊲OBu t ⊳4] containing four-coordinated Sc in edge-bridged
distorted tetrahedra as in Fig. 4.1. 102 With the less sterically demanding but more acidic
Figure 4.18 Chain structure of [Bi2( -
OC2H4OMe)4(OC2H4OMe)2]1