2 Homometallic Alkoxides

X-Ray Crystal Structures of Alkoxo Metal Compounds 275
isopropanol in [Hf2⊲OPr i ⊳8⊲Pr i OH⊳2] may be replaced by pyridine and the structure
of [Hf2⊲ -OPr i ⊳2⊲OPr i ⊳6⊲Pr i OH⊳⊲py⊳] shows that the edge-shared bi-octahedral
configuration is retained. The coordinated Pr i OH molecule is clearly hydrogen bonded
to the terminal isopropoxo group on the adjacent hafnium atom. 200 In the mixed ligand
complex [⊲Bu t O⊳3Hf⊲ -Cl⊳⊲ -OBu t ⊳⊲ -NHPMe3⊳Hf⊲OBu t ⊳3] the structure comprises
a confacial bi-octahedral configuration with three different bridging ligands. 198 A
mononuclear octahedral hafnium compound [Hf⊲OSiPh2OSiPh2OSiPh2O⊳2⊲py⊳2] was
obtained using the chelating hexaphenyl trisiloxane diolate ligands supplemented by
pyridine donors in the cis configuration. Eight-membered hafnasiloxane rings are
present in a distorted octahedral structure. 199 The Hf–O bonds trans to the pyridines
are shorter (1.961 ˚A) than those cis (2.014 ˚A).
2.10 Alkoxides of Vanadium, Niobium, and Tantalum
2.10.1 Vanadium
The structures of many alkoxo vanadium compounds have been reported but they
all contain oxo ligands and are accordingly dealt with in the chapter on metal oxo
alkoxides.
2.10.2 Niobium
The Nb(V) complex [Nb⊲OPr i ⊳3⊲diket⊳ (NCS)] (diket D PhCO⊲CH⊳COPh) has
a fac-octahedral structure. 202 The first homoleptic niobium alkoxide structure
determined was the dimeric pentamethoxide [Nb2⊲ -OMe⊳2⊲OMe⊳8] 203 and this
confirmed the edge-shared bi-octahedral configuration (Fig. 4.2) originally proposed
on the basis of variable-temperature 1 H NMR spectroscopy. 204 Two independent
centrosymmetrical molecules are present in the unit cell (data in Table 4.10, p. 281).
The niobium(IV) dichloride dimethoxide complexes [Nb2⊲ -OMe⊳2⊲OMe⊳2Cl4⊲L⊳2]
(L D MeOH, MeCN) also exhibit the edge-shared bi-octahedral structure. 205 In the
methanolate the four chlorines occupy the same plane as the Nb⊲ -OMe⊳2Nb double
bridge. Interestingly the methanol hydroxyls form intermolecular hydrogen bonds to
neighbouring chlorides. In the acetonitrile complex the MeCN ligands are in the
ClNb ( -OMe⊳2NbCl plane with methoxo groups trans to chlorines in the axial
positions. These diamagnetic molecules have d 1 –d 1 ,Nb–Nb single bonds (Nb–Nb,
2.781 ˚A, 2.768 ˚A) in contrast to the d 0 [Nb2⊲OMe⊳10] (Nb–Nb, 3.5 ˚A). A similar
structure is shown by the Nb(III) complex [Nb2⊲ -OPr i ⊳⊲ -Cl⊳⊲Pr i OH⊳4Cl4] which has
aNb⊲ -OPr i ⊳⊲ -Cl⊳Nb double bridge. 206 The Pr i OH ligands trans to chlorine (Nb–O,
2.09 ˚A, 2.13 ˚A) are significantly shorter than those trans to the bridging isopropoxo
group (Nb–O, 2.22 ˚A, 2.27 ˚A) and there is intramolecular H....Cl hydrogen bonding.
The formal double bond (NbDNb) character is reflected in the short niobium–niobium
distance (2.611 ˚A).
The [Nb2⊲ -OMe⊳3⊲OMe⊳6] anion has the confacial bi-octahedral structure in three
related salts [Mg⊲MeOH⊳6]2[Nb2⊲OMe⊳9]Cl3, [Mg⊲MeOH⊳6][Nb2⊲OMe⊳9]I.2MeOH
and [Na⊲MeOH⊳6][Nb2⊲OMe⊳9]. 207 The niobium–niobium bond distance is quite short
for a formally d 1 –d 1 single bond (Nb–Nb, 2.632–2.652, av. 2.640 ˚A) whilst the