The Diad Effect

In the lanthanide series, the equivalent values are much reduced by the retreat of the 4f electrons into the xenon core. This is so whether we consider processes that involve the condensation of gaseous ions, or conventional reactions. Table 1.3 includes data for the change 

These have been calculated from Caro s spectroscopic analyses [35]. The ligands come from opposite ends of the nephelauxetic series, so for a lanthanide reaction, A rep(irteg) should be relatively large. Even so, although it proves to be the largest contributor to the overall change, AEqs and AEso are significant Quantitative analyses of claimed examples of the tetrad effect must take such terms into account 

It is striking that, despite its small size, the tetrad effect was discovered before the diad effect. This is because the diad effect occurs in d-electron systems and is therefore masked by the orbital stabilization energies produced by the stronger ligand field. 

13 Morss, L. R. Standard Potentials in Aqueous Solution (Bard, A. (. Parsons, R. Jordan, J. eds.) Marcel Dekker, New York, 1985 p. 587. 

15 Johnson, D.A. Some Thermodynamic Aspects of Inorganic Chemistry, 2 ed Cambridge University Press, 1982 Chapter 6 and problems 6.6-6.9. 

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