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

Chapter 5
Under pseudo-first order conditions with [acacH] >>> [Mtot], the following equation for
the observed rate, kobs, can be obtained:
5.3.4 Discussion
k K [acacH]
k =
2 1
+k
obs 1+K [ acacH]
-2
1
89
(5.8)
The following conclusions are made from Scheme 5.1 and the derived rate law (Eq.
5.8):
The fast, first reaction (observed on the stopped-flow apparatus) can be
attributed to the formation of the Nb-acacH intermediate, [Nb(Cl)2(OMe)3
(acacH)]. When this fast reaction (k1 path) is considered in isolation, the
following simple, reversible equation can be derived:
[ ]
k =k L +k
obs 1 -1
(5.9)
It follows from Equation 5.9 that plots of kobs vs [acacH] should be linear with
the slope = k1 and the intercept = k-1. The equilibrium constant, K1, is defined
by Equation 5.10:
k
K =
1
1 k-1
(5.10)
The fact that the slower observed reaction, producing the chelated final acac
complex, yielded non-linear plots for kobs vs [acacH], indicates that the results
obtained here account for the overall reaction, requiring the data to be fitted
to the overall rate law (Eq. 5.8). The values of k2, k-2 and K1 should be
obtained from fits of kobs vs [acacH]. The equilibrium constant, K2, can be
determined from Equation 5.11:
k
K =
2
2 k-2
(5.11)