2 Homometallic Alkoxides

Heterometallic Alkoxides 211
analogous [HfAl2⊲OPr i ⊳10] shown in 1997 77 to conform in general to the structures
suggested above.
3.3.2 Hexaalkoxoniobate and -tantalate Derivatives
Compared to the tetra-alkoxoaluminates, the corresponding hexa-alkoxoniobate and
-tantalate derivatives of most of the metals are less stable. Of these two, the niobate
products tend to disproportionate more readily; this is reflected in the conductivity
curves obtained 211 in the titrations of niobium and tantalum pentaisopropoxide with
alkali (Na, K) isopropoxides in isopropyl alcohol (see Fig. 3.2).
All the MM 0 ⊲OR⊳6 derivatives (M D Li, Na, K; M 0 D Nb, Ta; R D Me, Et, Pr i ,
Bu t ) 35,36,212,213 are white solids soluble in the parent alcohols (solubility decreasing
from Li to K). If attempts are made to purify them by distillation/sublimation under
reduced pressure, the niobium derivatives tend to disproportionate more readily, yielding
the corresponding volatile niobium alkoxide and alkali alkoxide residues. By
contrast, MTa⊲OR⊳6 derivatives (M D Na, K; R D Me, Et) are stable up to ¾ 320 Ž C/
0.1 mm (the boiling points of Ta⊲OMe⊳5 and Ta⊲OEt⊳5 are 180 Ž C/10 mm and 202 Ž C/
10 mm, respectively).
Hexaisopropoxotantalates of Li, Na, K, and Cs tend to sublime around 220 Ž C under
0.1 mm pressure and all the corresponding tertiary butoxy analogues, MfM 0 ⊲OBu t ⊳6g
(M D Li, Na, K, Cs; M 0 D Nb, Ta) can be volatilized at lower temperatures around
110–120 Ž C/0.1 mm. The thermal stability of the MTa⊲OR⊳6 derivatives appears to
follow the orders: Li > Na > KandOBu t > OPr i > OEt ³ OMe. Molecular weight
studies also appear to indicate the same pattern.
Specific conductivity (× 10 6 ohm −1 cm −1 )
30
20
10
0
IV
II
I III
0.4 0.8 1.2 1.6 2.0 2.4
Molar ratio M(OPri )
M′(OPri ) 5
Figure 3.2 Titration between
M 0 ⊲OPr i ⊳5 and M⊲OPr i ⊳. Curve I:
(ž) NaOPr i M/4.77 vs Ta⊲OPr i ⊳5
M/93.74; Curve II: (ð) KOPr i M/4.96
vs Ta⊲OPr i ⊳5 M/93.74; Curve III:
(Ž) NaOPr i M/4.77 vs Nb⊲OPr i ⊳5
M/94.24; Curve IV: (�) KOPr i
M/4.98 vs Nb⊲OPr i ⊳5 M/94.91.