2 Homometallic Alkoxides

58 Alkoxo and Aryloxo Derivatives of Metals
these derivatives. In fact, the molecular weight determinations of sodium perfluoro
tert-butoxides 442 and those of the tert-butoxide derivatives 443 of potassium, rubidium,
and caesium have indicated these derivatives to be tetrameric in nature. Since the I
effect of perfluoroalkyl groups should enhance the polarity of the M–O bonds, the
relatively high volatility and solubility of these perfluoroalkoxides might be due in
part to the weak intermolecular forces between the fluorocarbon groups.
During the preparation of trifluoromethoxides of the alkali metals involving reaction
of alkali metal fluorides with carbonyl fluoride, Redwood and Willis 444 concluded that
because of the ionic character of the fluoromethoxide derivatives M C ⊲OCF3⊳ the stable
trifluoromethoxide derivatives would be formed only with those metals which have a
greater atomic size. Thus trifluoromethoxides of potassium, rubidium, and caesium
arestablebelow 20 Ž C but those of lithium and sodium could not be isolated. The
trifluoromethoxides of potassium, rubidium, and caesium decompose at 80 Ž C under
reduced pressure. From the curves of Fig. 2.2, it is clear that the rate of decomposition
is slowest for the caesium derivative, which has the largest cation. Thus the order of
stability of these alkali metal trifluoromethoxides is CsOCF3 > RbOCF3 > KOCF3.
Further, the thermal stability of the product decreases with increasing chain length of
the fluoroalkoxide. This is because, although the oxygen atom is negatively charged in
the trifluoromethoxide ion, the higher electronegativity of three fluorine atoms would
tend to delocalize the charge over the ion and thus bring about comparatively greater
stability. However, with pentafluoroethoxide or heptafluoropropoxide, which contain
more than one carbon atom, the stability is reduced because the carbon atom is less
electronegative than fluorine. Therefore, for the fluoroalkoxide derivative of the same
metal the observed stabilities are in the decreasing order: 445 MOCF3 > MOCF2CF3 >
MOCF2CF2CF3 > MOCF⊲CF3⊳2 which can be represented graphically by Fig. 2.3.
3.2.4 Alkoxides of Group 2 and 12 Metals
The primary alkoxide derivatives of alkaline earth and other metals of group 2 are
generally insoluble nonvolatile compounds whereas their highly branched alkoxides
Decomposition (%)
160
120
80
40
0
30
KOCF 3
RbOCF 3
CsOCF 3
60 90
Time (min)
Figure 2.2 Decomposition curve of alkali
trifluoromethoxides at 80 Ž C.