Course C21J; Inorganic Reaction Mechanisms
Problem Paper # 2

October 27, 2003

1. The mechanism of ligand substitution in square planar complexes is represented by a two term rate law. Write the mechanism and derive the rate law. Discuss the evidence in support of the mechanism.

2. What are the factors that could affect the rate of square planar substitutions? Justify your proposal.

3. The rates and equilibrium constants for the aquation reaction of [Co(NH3)5X]2+ are
X NCS- N3- Cl- I- NO3-
k/s-1 4.1x10-10 2.1x10-9 1.7x10-6 8.3x10-6 2.7x10-5
K/mol-1dm3 3.7x10-4 1.2x10-3 0.9 8.3 12

Plot the data and explain the result in terms of a possible mechanism.

4. Explain why the rate of aquation of [Co(NH3)5Cl]2+is about 106 times slower than the corresponding base hydrolysis.

5. The following rates were obtained for the anation of [Co(NH3)5OH2]3+ by a series of nucleophiles, L, at 45°C in aqueous solution.

L 106 k1/s-1 k1 / ke
NCS- 16 0.16
H2PO4- 15 0.15
Cl- 21 0.21
N3- 100 1.00

k1 is a limiting rate constant and keis the rate of water exchange. Suggest a mechanism for the anation reaction on the basis of the above results.
6). In 0.5 mol dm-3 NaOH 96% of [(NH3)5CoCl]2+that undergoes base hydrolysis must be present in the pentaammine form. Calculate the acid dissociation constant of the complex ion when the ionic product of water, Kw, is 1.0x10-14. Comment on the validity of a Dcb mechanism on the basis of your calculation and the observations described above.

7). The kinetics of reactions of [(NH3)5CoCl]2+at different concentrations of NaOH have been studied at 25 °C. The following data were obtained:

[OH- /mol dm-3 0.1 0.2 0.3 0.5
kobs/s-1 0.085 0.170 0.225 0.42

Discuss possible mechanisms on the basis of the above results.

8). The rates of base hydrolysis of octahedral complexes are generally retarded by the addition of H2O2and a fractionation factor, f, of 1.00 is usually found, where

f = (16O/18O) in product / (16O/18O) in solvent.

Explain the implication of these observations on your choice of the mechanism.

Copyright © 2003 by Tara Dasgupta, all rights reserved.

Maintained by Prof. Robert J. Lancashire,
The Department of Chemistry, University of the West Indies,
Mona Campus, Kingston 7, Jamaica.
Created October 2003. Links checked and/or last modified 27th October 2003.