[1][2] It refers to a decrease in the Racah interelectronic repulsion parameter, given the symbol B, that occurs when a transition-metal free ion forms a complex with ligands.
The presence of this effect highlights the disadvantages of crystal field theory, which treats metal-ligand interactions as purely electrostatic, since the nephelauxetic effect reveals the covalent character in the metal-ligand interaction.
The decrease in the Racah parameter B indicates that in a complex there is less repulsion between the two electrons in a given doubly occupied metal d-orbital than there is in the respective Mn+ gaseous metal ion, which in turn implies that the size of the orbital is larger in the complex.
Experimentally, it is observed that size of the nephelauxetic parameter always follows a certain trend with respect to the nature of the ligands present.
The list shown below enlists some common ligands (showing increasing nephelauxetic effect):[3] Although parts of this series may seem quite similar to the spectrochemical series of ligands - for example, cyanide, ethylenediamine, and fluoride seem to occupy similar positions in the two - others such as chloride, iodide and bromide (amongst others), occupy very different positions.