2 Homometallic Alkoxides

396 Alkoxo and Aryloxo Derivatives of Metals
unstable. The mononuclear oxo vanadium triethanolaminate [OVf⊲OCH2⊳3Ng] gave
a parent molecule ion 83 whereas the oxoniobium trialkoxides [ONb⊲OR⊳3] (RD Me,
Et, Pr i ,Bu n ) gave evidence for disproportionation to Nb⊲OR⊳5 and NbO2⊲OR⊳ in
the mass spectrometer. 84 On the other hand [Ta2O⊲OPr i ⊳8⊲Pr i OH⊳] gave evidence of
dinuclear fragment ions 85 and both [Me⊲C2Me4O2⊳Re⊲O⊳⊲ -O⊳Re⊲O⊳⊲O2C2Me4⊳Me]
and [Me3Re⊲O⊳⊲ -O⊳Re⊲O⊳Me3] gave parent ions. 86 Interestingly the oxo-bridged
Fe(III) complex [(L)FeOFe(L)] containing the pentadentate ligand L D
[MeCfCH2N ž žCMe.CH2C⊲CF3⊳2gO2⊲CH2NH2⊳] gave⊲L⊳Fe C and ⊲L⊳FeO C fragment
ions due to unequal splitting of the dinuclear species. 87
The tetranuclear lead oxo-tert-butoxide [Pb4O⊲OBu t ⊳6] gave a substantial parent
molecular ion but the most intense lead-containing species was [Pb4O⊲OBu t ⊳5] C
showing that the Pb4O5 core was stable. 22
4.4 Magnetic Properties
Some of the transition metal oxo-alkoxide cluster molecules have shown interesting
magnetic properties in view of the current theory of magnetic exchange mechanisms.
Some examples of systems subjected to detailed magnetic studies are:
⊲NEt4⊳2[Mn10O2Cl8f⊲OCH2⊳3CMeg6], 88 [Mn13O8⊲OEt⊳6⊲O2CPh⊳12], 89
[fFe6O2⊲OMe⊳12⊲tren⊳2g⊲O3SCF3⊳2] (trenD ˇˇ 0 ˇ 00 -triaminotriethylamine⊳[N⊲CH2CH2NH2⊳3],
90 [NMe4⊳2[Fe6O⊲OMe⊳3Cl6f⊲OCH2⊳3CMeg3], 91
[Fe12O2⊲OMe⊳18Cl0.7⊲O2CCH2Cl⊳5.3⊲CH3OH⊳4], 92
Na[Fe4O⊲OH⊳fOCH[CH2N⊲CH2CO2⊳2]2g2fO2CCH⊲Me⊳NH3g2], 93 and
[Zr4Cu4O3⊲OPr i ⊳18]. 129
5 X-RAY CRYSTAL STRUCTURES
In the 1978 book Metal Alkoxides 1 only three authentic X-ray crystal structures of
metal oxo-alkoxides were described, namely [V2⊲O⊳2⊲OMe⊳6], [Ti7O4⊲OEt⊳20], and
[Nb8O10⊲OEt⊳20]. Since then there has been an explosive growth in the number of
structure determinations with examples of 6-O, 5-O, 4-O, 3-O, -O, and Ot
oxo-environments. 4
In the metal (and heterometal) oxo-alkoxides the metals adopt coordination numbers
that are common in the parent metal alkoxides, but the presence of the oxo ligand,
with its ability to bond to six metals ( 6) in the centre of a molecule, is often structure
determining. Since the emphasis in this section is on the behaviour of the oxo ligand
rather than the metal the structures of heterometal oxo-alkoxides will be considered
together with those of the metal oxo-alkoxides. Owing to the very large number of
structures known, only a few will be considered in detail and the data on the majority
will be tabulated.
5.1 Alkali-metal Containing Oxo-alkoxides (Including Heterometallic
Compounds)
Although no alkali metal oxo-alkoxide has yet been structurally characterized the structure
of the remarkable heteroalkali metal complex [Li8K2⊲O⊳⊲OBu t ⊳8⊲Me2NC2H4NMe2⊳2]