A solution and solid state study of niobium complexes University of ...

Chapter 2
A slower rate was observed in benzene than in ethanol. The reason for this is that
alcohol favours the conversion of the dimer to the monomer. 87 The absorption curves
illustrate the fast absorption of CO2 during the first few minutes, followed by a slow
continuous absorption over several hours. This is displayed by both the ethanol and
benzene solutions. This supports the hypothesis that the carboxylation of the
monomer is faster than the dissociation of the dimer.
Vcarb = k2[Nb(OEt)5][CO2] (2.15)
Second order kinetics was determined for the carboxylation reaction of [Nb(OEt)5]2
(Eq. 2.15). k2 was calculated as 1.19 x 10 -1 l.mol -1 .s -1 for the first part of the
absorption curve, that is the carboxylation step. The same value was determined for
both the ethanol and benzene solutions. The second part of the curve was used to
calculate the dissociation process. The k1 value in ethanol was determined as 1.15 x
10 -5 s -1 and that for benzene was determined as 0.97 x 10 -5 s -1 .
The insertion of CO2 into the Nb-OEt bond was found to be reversible. The CO2 was
released by the hemicarbonate under vacuum. This was confirmed by the fact that
the IR signal due to the carbonate group disappeared and the starting complex
([Nb(OEt)5]2) was quantitatively regenerated.
2.9 Conclusion
Most of the work done on niobium focuses on the +5 oxidation state. The syntheses
of these complexes are complicated due to the metal’s affinity to readily form oxides.
A few niobium complexes have also been characterized to contain oxo bridges.
Relatively few papers have been published on the kinetics of niobium complexes.
From literature, it is clear that there is a significant need for research in the
separation of niobium and tantalum without the use of HF.
87 D. C. Bradley, C. E. Holloway, J. Chem. Soc., 219, 1968.
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