2 Homometallic Alkoxides

398 Alkoxo and Aryloxo Derivatives of Metals
tetrahedrally bonded in K2Sb2O units which have significantly shorter metal–oxo bond
lengths than in the 6-O complex. 99
5.2 Magnesium and Alkaline Earth-containing Oxo-alkoxides (Including
Heterometallic Compounds)
5.2.1 Magnesium and Calcium
In the magnesium molybdenum(V) oxo-methoxide [Mg 2 Mo2⊲O⊳2⊲OMe⊳10⊲MeOH4⊳]
compound the oxo-ligands are terminally bonded to the molybdenums. 100 Both
metals are octahedrally coordinated, with an Mg 2 Mo2O16 structure similar to that
of the Ti4⊲OR⊳16 molecules. The structure is preserved in the Mo(VI) derivative
[Mg 2 Mo2⊲O⊳4⊲OMe⊳8⊲MeOH⊳4] in which the terminally bonded oxo-ligands are cis
to each other. 100
Inthehexanuclearcalciumoxo-ethoxide[Ca6⊲ 4-O⊳2⊲ 3-OEt⊳4⊲OEt⊳4⊲EtOH⊳6].8EtOH
the 4-oxo ligands are bonded to octahedrally coordinated calciums in a structure comprisingtwoCa4O4
cubaneunitssharingacommonCa2O2 face. 101 Somehexanuclearbarium
oxo-alkoxides have also been characterized.
5.2.2 Barium
The tert-butoxide [H3Ba6⊲ 5-O⊳⊲OBu t ⊳11⊲OCEt2CH2O⊳⊲thf⊳3] has a square-based
pyramid of six-coordinated bariums with a 5-oxo (or 5-OH) and the sixth
barium is attached in a five-coordinated mode. 102 In [H4Ba6⊲ 6-O⊳⊲OC2H4OMe⊳14]
the 6-oxo ligand is encapsulated within the octahedron of barium atoms,
which are all eight-coordinated with the methoxymethoxo ligands exhibiting
three different modes of bonding. 103 Again in the pentanuclear species
[Ba5⊲ 5-O⊳f 3-OCH⊲CF3⊳2g4f -OCH⊲CF3⊳2g4f⊲CF3⊳2CHOHg⊲thf⊳4⊲H2O⊳] thereisa
square pyramid of Ba atoms bonded to either a 5-oxo or 5-hydroxo ligand. 104 Four
of the bariums are six-coordinated and the fifth is seven-coordinated.
The centrosymmetrical heterometallic oxo-alkoxide-diketonate molecule [Ba2Y4⊲ 6-
O⊳⊲ 3-OEt⊳8⊲Bu t COCHCOBu t ⊳6] also contains an octahedrally encapsulated 6-oxoligand.
105
The structures of several barium titanium oxo-alkoxides have been determined due
to their interest as precursors for the production of BaTiO3. The methoxo complex
[Ba2Ti4⊲ 3-O⊳⊲ 3-OMe⊳6⊲ -OMe⊳2⊲OMe⊳10⊲MeOH⊳7] has a remarkable structure
(Fig. 5.3) containing nine-coordinated Ba ions and octahedral Ti atoms. 20 Even more
remarkable is the structure of the barium titanium oxo-isopropoxide which contains
two slightly different molecules [Ba4Ti4⊲ 4-O⊳4⊲ 3-OPr i ⊳2⊲ -OPr i ⊳8⊲OPr i ⊳6⊲Pr i OH⊳4]
and [Ba4Ti4⊲ 4-O⊳4⊲ 3-OPr i ⊳2⊲ -OPr i ⊳9⊲OPr i ⊳5⊲Pr i OH⊳3] in the unit cell. 19,106 Each
molecule has a Ba4O4 distorted cubane core with each oxo ligand bonded to a
titanium atom. All of the bariums are seven-coordinated and the titaniums are
five-coordinated. The structure of [Ba4Ti4⊲ 4-O⊳4⊲OPr i ⊳16⊲Pr i OH⊳3] has also been
determined independently. 107
The methoxymethoxo ligand stabilized the giant barium titanium oxo-alkoxide
[Ba4Ti13⊲ 5-O⊳6⊲ 3-O⊳12⊲OC2H4OMe⊳24]. The central octahedral Ti is bonded to the
six trigonal pyramidal 5-oxo ligands which are each bonded axially to two bariums
and equatorially to two of the external titaniums, which are also bonded to two 3-oxo