Heavy electron manifestation

The above effects manifest themselves both in electron transfer reactions and in reactions involving the transfer of heavy particles. However, before discussing these effects in electron transfer reactions, we will consider some problems arising in the reference model. 

As regards (b), the pioneering work of Lundqvist and Brandt is already summarized in ref. 10 (p. 121). This has led to exciting developments relating to collective manifestations of electron interactions in some specific shells of heavy atoms. But so far this has not been posed at any quantitative level in terms of the electron density and would take us too far outside the scope of this article. 

Molecules that contain heavy elements (in particular 5d transition metals) play an important role in the photochemistry and photophysics of coordination compounds for their luminescent properties as well as for their implication in catalysis and energy/electron transfer processes. Whereas molecular properties and electronic spectroscopy of light molecules can be studied in a non-relativistic quantum chemical model, one has to consider the theory of relativity when dealing with elements that belong to the lower region of the periodic table. As far as transition metal complexes are concerned one has to distinguish between different manifestations of relativity. Important but not directly observable manifestations of relativity are the mass velocity correction and the Darwin correction. These terms lead to the so-called relativistic contraction of the s- and p- shells and to the relativistic expansion of the d- and f- shells. A chemical consequence of this is for instance a destabilisation of the 5d shells with respect to the 3d shells in transition metals. 

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