2 Homometallic Alkoxides

X-Ray Crystal Structures of Alkoxo Metal Compounds 243
bulky Ph3SiO ligands a mononuclear pseudo-tetrahedral complex [Al⊲OSiPh3⊳2(acac)]
is formed. 64
2.3.2 Gallium
Although no structure of a homoleptic alkoxide of gallium has yet been reported some
five-coordinated species involving functionalized alkoxo ligands are known. 66–68 The
hydrido complex [GaHf⊲OC2H4⊳2NMeg]2 66 contains the chelating ligand N-methyl
diethanolaminate and is dimerized through bridging by one of the oxygens, giving
a distorted TBP configuration for each gallium with the hydrides each occupying
an equatorial site. Similarly in the bridging N,N-dimethylethanolaminates [GaH2⊲ -
OC2H4NMe2⊳]2 and [GaMe2⊲ -OC2H4NMe2⊳]2 the galliums are in distorted TBP configurations.
67 The pyridylmethoxo ligand also bridges in [Me2Gaf -OCH2⊳C5H4N⊳g]2
to form a five-coordinated dimer. 68 All of these complexes contain the Ga2O4 fourmembered
ring (data are presented in Table 4.3). In [Bu t 2 Ga⊲ -OBut ⊳]2 the bridging
tert-butoxo groups from a centrosymmetrical dimer with distorted tetrahedral galliums.
69 Replacing the tert-butoxo ligand with the bulkier triphenylmethoxo ligand
gave rise to the mononuclear three-coordinated compound [Bu t 2 Ga(OCPh 3⊳]. 70 Similar
dimeric species [Ga⊲ -OCH2SiMe3⊳⊲CH2SiMe3⊳2]2 71 and [Ga⊲ -OBu t ⊳H2]2 62 have
also been structurally characterized.
2.3.3 Indium and Thallium
To date two structures of organoindium alkoxides have been reported, [In⊲ -
OEt⊳Bu t 2 ]2 72 and [In ( -OCH2SiMe3⊳⊲CH2SiMe3⊳2]2. 71 Even less structural data is
available on thallium alkoxides. The tetranuclear [Tl4⊲ 3-OMe⊳4] wasshowntohave
a distorted cubane Tl4O4 framework (Fig. 4.6) although the Tl–O bond distances
were not determined. Recently the remarkable chain copolymer [WCl2(PMe 2Ph⊳4,
(TlOCH 2CF3⊳4]n has been shown to crystallize as two polymorphs. 74 Weak Tl....Cl
interactions and charge-transfer are believed to support the structure in which the
cubane Tl4O4 unit is present.
2.4 Alkoxides of p-Block Elements: Ge, Sn, Pb
2.4.1 Germanium
Most of the structures of quadrivalent germanium alkoxo compounds reported to date
involve the triethanolaminate ligand [RGef(OC2H4⊳3Ng] (R D Et, naphthyl, 1
2CH2, Br, But , NCS). 75–77 In each case the germatrane structure (Fig. 4.16) contains
a five-coordinated Ge in a TBP configuration with the three oxygen atoms in
the equatorial plane (data in Table 4.4, pp. 248–249). Other Ge(IV) compounds
studied are the six-coordinated [Ge(OMe) 2(porphyrinate)] 78 and the four-coordinated
[Ge(OEt)HfCH(SiMe3⊳2g2]. 79
The first homoleptic Ge(II) alkoxide to be structurally determined was the mononuclear
[Ge(OCBu t 3⊳2] containing two-coordinated Ge owing to the extreme bulkiness
of the ligands. A noteworthy feature is the acute O� GeO angle of 85.9Ž . With
less demanding ligands as in [Ge2⊲ -OBu t ⊳2Cl2] a dimer is formed with trigonal