2 Homometallic Alkoxides

254 Alkoxo and Aryloxo Derivatives of Metals
Figure 4.22 Structure of [Y3( 3-
OC2H4OMe)2(OC2H4OMe)3(acac)]
(acac D acetylacetonate; H atoms
omitted).
the seven-coordinated yttrium has a OBu t group and a chloride in the axial positions
with the five THF ligands occupying the equatorial positions. 110 Some structures
have also been determined on mono- and bis-cyclopentadienyl yttrium alkoxo
compounds. The dimeric species [⊲C5H4SiMe3⊳4Y2⊲ -OMe⊳2], 111 [(C5Me5⊳2Y2⊲ -
OBu t ⊳2(OBu t ⊳2], [(C5H5⊳2Y2⊲ -OBu t ⊳2], [(C5H4SiMe3⊳2Y2⊲ -OBu t ⊳2(OBu t ⊳2] and
[(C9H7⊳2Y2⊲ -OBu t ⊳2(OBu t ⊳2] all contained Y( -OR⊳2Y bridges (data in Table 4.6,
pp. 256–257). 112
The bridging propensity of the alkoxo ligand can be reduced both sterically by using
bulky alkyl groups and electronically by using electronegative alkyl groups, which
reduce the electron density on the alkoxo oxygen donor atom. Both factors are operatinginthefluorinated
tertiary alkoxo groups, and the mononuclear yttrium compounds
[YfOCMe⊲CF3⊳2g3(thf) 3]and[YfOCMe⊲CF3⊳2g3⊲diglyme⊳] (Fig. 4.23) were obtained
using the hexafluoro-tert-butoxo (hftb) ligand. 113 Both complexes are octahedral with
the facial configuration. The tris-THF complex exhibits the same structure as the previously
reported tris-triphenylsiloxo complex [Y(OSiPh 3⊳3(thf) 3] (Fig. 4.24). 114
The homoleptic tris-triphenylsiloxide forms an unsymmetrical dimer [Y2⊲ -
OSiPh3⊳2⊲OSiPh3⊳4] with distorted tetrahedral yttrium atoms. 115 Interestingly a mononuclear
five-coordinated adduct [Y(OSiPh 3⊳3⊲OPBu n 3 ⊳2] was formed with tri-n-butylphosphine
oxide ligands occupying the axial positions in a TBP configuration. 116 In the
salt [K(dme) 4] C [Y(OSiPh 3⊳4⊲dme⊳] the yttrium mono-anion adopts a distorted octahedral
configuration. 116 The dimethyl tert-butylsiloxo ligand gave rise to the mononuclear
adduct [Y(OSiMe 2Bu t ⊳3(thf) 3] in the presence of THF but in the presence of
excess silanol the novel unsymmetrical dinuclear species [⊲Me2Bu t SiOH⊳⊲Me2Bu t SiO⊳2
Y⊲ -OSiMe2Bu t ⊳2Y⊲OSiMe2Bu t ⊳2] was formed. 116 One yttrium is in a distorted tetrahedral
configuration while the other one is approximately trigonal bipyramidal, and
this is reflected in the bond lengths (Table 4.6).