Reaction kinetics of coordination compounds
In the majority of chapters of this book the concern has been with an understanding of the properties of individual molecules, properties which are regarded as essentially time-independent. However, no less important are the chemical reactions of these mo
- PDF / 2,873,917 Bytes
- 28 Pages / 439.37 x 666.142 pts Page_size
- 9 Downloads / 231 Views
+ [Co(NH 3 ) 5 CI]2+
-+
[Co(NH 3 ) 5 H2 0] 3 +
+ Cl -
Because [Co(NH 3 ) 5 Cl]2+ salts are soluble in few other solvents, the study will probably be carried out with water as a solvent, as well as reactant. In this case, each [Co(NH 3 ) 5 Cl]2+ ion in solution will undergo a constant battering from water molecules incident from all directions. No doubt, the coordinated ammonia molecules will move in response, although perhaps restrained somewhat by hydrogen bonding to the solvent. For the reaction to occur, a Co-Cl bond has to be broken and this could be because vibrations of the coordinated ammonia ligands temporarily expose the Co-Cl bonding electrons to attack by a water molecule. Equally, it could be that a transient, strong hydrogen bond is formed between the Clligand and a water molecule, so facilitating breaking of the Co-Cl bond. Again, Co-Cl bond breaking could be dependent on much of the vibrational energy within the molecule localizing itself briefly in the Co-Cl bond stretching mode. Alternatively, some combination of all three factors could be involved. It seems likely that a multitude of slightly different reaction pathways exist and, since we study a large number of molecules simultaneously, all we can
S. F. A. Kettle, Physical Inorganic Chemistry © S. F. A. Kettle 1996
318
1
Reaction kinetics of coordination compounds
measure is some sort of average. Hopefully, there will be only one, clearly defined, lowest energy reaction route and the average will be dominated by this and its minor deviants. However, the three contributions listed above (and others could be added) will presumably have different temperature characteristics and so the (average) reaction route will also be slightly temperature dependent. Fortunately, this complication can usually be ignored -it seems to be of lesser importance than experimental error. Attention therefore focuses on a single reaction pathway and, in particular, the different potential energy profiles associated with alternative pathways. The task of the worker in the area is to use the experimental data, and, in particular, rate laws, to deduce the most probable reaction pathway. It is not an easy task. As will be seen, the existence of pre-equilibria (which mean that the real reactant species is present in much lower concentration than expected), the involvement of the solvent (which, because its concentration scarcely changes, will not be evident from the rate law), the ability to work over limited temperature and concentration ranges, all impose problems. Fortunately, more techniques are becoming available; thus, the ability to study reaction rates in solution as a function of the external pressure applied to the solution offers information as to whether the reaction pathway involves compression or extension of the reactant species and, thus, insights into its molecularity. Only in one area can theory be said to have led experiment. This is in one aspect of oxidation-reduction, electron-transfer, reactions, a topic to which we shall return. First, howeve
Data Loading...
