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

5.4 Conclusion
Chapter 5
The values of K1 obtained from two separate experiments are the same within
experimental error. This is a further indication that the reaction scheme and resulting
rate law provides a good understanding of the formation reactions studied here. The
value of K2 is three orders of magnitude smaller than K1 (e.g. at 25.0 °C, K1 = 1975
(201) M -1 vs K2 = 2.0 (5) M -1 ). This is to be expected since the rate-determining ring-
closure is slow. Similar examples, where this has been observed, are known for
bidentate complexes. 7
The relative rate of the ring-closure is best illustrated by comparing k1 to k2. The rate
data cannot be compared simply by looking at the results in Table 5.2, since the k2
rate constant is only measured in s -1 . The rate constant, k1 (at 25.0 ° C), is converted
to a first-order rate constant by multiplying k1 (237 (4) M -1 s -1 ) with [acacH] (0.05 M)
and adding k-1 (0.12 (1) s -1 ). When this value of 11.97 (2) s -1 is compared with k2 (3.7
(3) x 10 -5 ), a difference of 6 orders of magnitude is obtained.
The difference between k2 and k-2 is between 1.5 and 2 times at all the temperatures
studied. This is also a very good indication of the small value obtained for the
equilibrium constant, K2.
Comparison of the kinetic data obtained for this study with that of the corresponding
tantalum study 8 yielded the following:
Similar rate laws were obtained for both studies, except that no k-2 step was
observed for the tantalum study.
The values obtained for the rate constants, k1 and k2, of the tantalum study
(316 (9) M -1 s -1 ; 4.43 (7) x 10 -5 s -1 ) was slightly higher than that obtained for
the niobium study (237 (4) M -1 s -1 , 3.7 (3) x 10 -5 s -1 ) at 25.0 °C.
The equilibrium constant, K1, for both studies are, within experimental error,
similar at ± 2000 M -1 .
The high positive value obtained for ∆S ‡ (k1), points strongly towards a dissociative
type mechanism for the first step, as expected for octahedral substitution reactions.
7 H. J. van der Westhuizen, R. Meijboom, M. Schutte, A. Roodt, Inorg. Chem., 49, 20, 9599, 2010.
8 R. Koen, H. G. Visser, A. Roodt, M.Sc. Thesis, University of the Free State, 2012.
93