A General Principle of Lanthanide Chemistry

The second class of reaction is that of processes in which the 4f electrons are conserved. The obvious examples are the complexing reactions of tripositive lanthanide ions. Here the irregularities due to changes in inter-electronic repulsion almost entirely disappear. We then get the slight smooth energy change whose consequences were so familiar to 19 century chemists, who struggled vdth the separation problem. 

In many cases, lanthanide reactions can either be assigned exclusively to one of these two classes, or they show deviations that the classification makes understandable. In Fig. 1.2, we plot the values of AH for the complexing of the tripositive aqueous ions by EDTA (aq), a reaction in which the 4f electrons are conserved. The irregularities are negligible at the chosen scale. Also shown are the values of AH (MCl3,s) which refer to  

for nearly all of the elements, the number of 4f electrons in the metallic state and in the trichloride is the same, so we expect a largely smooth energy 

The principle introduced above is best exploited by classifying lanthanide compounds not by oxidation state, but by the number of 4f electrons at the metal site. For example, the reaction 

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A general principle of lanthanide chemistry stated by Johnson in 1980 is that in processes which do not involve changes in the number of f electrons, the lanthanides behave similarly. While this holds for many properties of the lanthanides, there are some important exceptions where trends are not regular. For example, trends in the formation constants of complexes of Ln + ions often show irregularities when there is no change in oxidation state of the metal. Many seemingly irregular trends can be rationalized by a more detailed analysis of the processes involved. ... 

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