Ion polarisation

The precise sequence of events depends upon the combination of ligands and metal centres involved, but the key step involves a C-H bond-breaking reaction. The reaction may be viewed as a consequence of metal ion polarisation of the ligand increasing the acidity of the relevant C-H bond. Loss of a proton yields a carbanion, which undergoes an electron transfer reaction with the metal centre to yield a radical and lower oxidation state metal ion (free or co-ordinated). It must be emphasised that this is purely a formal view of the reactions. 

The bromine molecule becomes bromonium ion polarised as it approaches the alkene. 

In our model of an ionic lattice, we have thought of the ions as being spherical in shape. This is not always the case. In some cases, the positive charge on the cation in an ionic lattice may attract the electrons in the anion towards it. This results in a distortion of the electron cloud of the anion and the anion is no longer spherical (Figure 19.7). We call this distortion ion polarisation. The ability of a cation to attract electrons and distort an anion is called the polarising power of the cation. 

Figure 19.7 Ion polarisation. A small highly charged cation can distort the shape of the anion.

We can explain this trend using ideas about ion polarisation ... 

One of these ion properties is their polarisability. Ninham and co-workers proposed that polarisability was the key missing parameter further to ion sizes and charges. Also Jungwirth and Tobias based their remarkably predictive simulation results for ions near surfaces on potentials that took ion polarisabilities into account. Therefore, for several years after these papers, people were optimistic that the mystery of the... 

Hofmeister series could finally be unravelled with a simple parameter. Now, we are much less optimistic. In 2004, Kunz etal. showed that a simplistic use of the ion polarisabilities is misleading for the interpretation of bulk effects. It turned out that it is not possible to reproduce surface tensions and osmotic coefficients with the same parameters. 

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