2 Homometallic Alkoxides

266 Alkoxo and Aryloxo Derivatives of Metals
was formed using the dimethyl-tert-butyl siloxo ligand with DME as supporting ligand
in [Yb2⊲ -OCMe2Bu t ⊳2⊲OCMe2Bu t ⊳2(dme) 2]. 142
2.7.8 Lutetium
Lutetium(III) forms a novel unsymmetrical dinuclear molecule [Lu2( -
OCMe2CH2OMe)3(OCMe2CH2OMe)3] in which one Lu is seven-coordinated and the
other six-coordinated (Fig. 4.33). 143 The six-coordinated Lu is bonded to three bridging
ligands (one of which is chelating) and two terminals whilst the seven-coordinated
metal is bonded to three bridging (two of which are chelating) and one chelating
ligand.
2.8 Alkoxides of the Actinides
2.8.1 Thorium
Although thorium(IV) alkoxides have been known for many years it was only
in 1992 that the X-ray crystal structure of a homoleptic thorium compound was
reported. Using the bulky di-isopropylcarbinolate ligand the dimeric compound [Th2⊲ -
OCHPr i 2 ⊳2⊲OCHPr i 2 ⊳6] was obtained in which each Th is in a distorted TBP
coordination. 144 The monomeric tert-butoxide [Th⊲OBu t ⊳4⊲C5H5N⊳2] has a distorted
octahedral configuration with pyridine ligands occupying cis positions. 145 With the less
sterically demanding Et2CHO ligand an edge-shared bi-octahedral complex [Th2⊲ -
OCHEt2⊳2⊲OCHEt2⊳6⊲C5H5N⊳2] was obtained. Each thorium is coordinated to one
pyridine ligand located cis to the bridging alkoxo groups in a centrosymmetric
molecule. 146 With the isopropoxo ligand the remarkable tetranuclear complex
[Th4⊲ -OPr i ⊳6⊲OPr i ⊳10⊲C5H5N⊳2] was obtained (Fig. 4.34). 146 The thorium ions
are all octahedrally coordinated forming a zigzag chain through cis pairs of
bridging isopropoxides. The two inner thoriums each have cis pairs of terminal
isopropoxo groups whilst the end thoriums each have a pyridine ligand acting
as chain blocker. The series of edge-sharing bi-octohedral dimers give rise to a
centrosymmetrical molecule. The edge-sharing bi-octahedral structure was also found
in the dimer [Th2⊲ -OPr i ⊳2⊲OPr i ⊳2I4⊲Pr i OH⊳2] where the bridging isopropoxo group
is unsymmetrical with a short Th–O (2.327 ˚A) trans to an iodide and the longer Th–O
Figure 4.33 Structure of [Lu2⊲ -
OCMe2CH2OMe⊳3⊲OCMe2CH2OMe⊳3]
(H atoms omitted).