Nephelauxetic Series

The term nephelauxetic refers to the expansion of the electron cloud of d-orbitals in metal ions when they form complexes with ligands, which leads to a decrease in inter-electronic repulsion. This electron cloud expansion effect may occur for one (or both) of two reasons. One is that the effective positive charge on the metal has decreased. Because the positive charge of the metal is reduced by any negative charge on the ligands, the d-orbitals can expand slightly. The second is the act of overlapping with ligand orbitals and forming covalent bonds increases orbital size, because the resulting molecular orbital is formed from two atomic orbitals.

The nephelauxetic series is a list of ligands and metal ions ordered according to their nephelauxetic effect. This effect is very frequently used for transition metal complexes and refers to a decrease in the Racah interelectronic repulsion parameter. Racah parameter(B) is measured when a transition metal free ions forms a complex with different types of ligands.
The decreasing value of the Racah parameter indicates less repulsion between two electrons in the d orbitals of a metal complex and the orbital is larger in the complex. It is important in determining the nephelauxetic effect/series.

The nephelauxetic effect is represented in terms of nephelauxetic parameter(β) which is given by
The value of β is always less then one and it decreases with increasing delocalization. In general B' = ~ 0.8B.

Nephelauxetic series of ligands

The common ligands can be arranged in the order of their Nephelauxetic effect, in the form of a series called Nephelauxetic series of ligands. it is similar to the spectrochemical series.
F⁻ > H₂O > urea > NH₃ > en > SCN⁻ > Cl⁻ > CN⁻ > Br⁻ > I⁻

Nephelauxetic series of metal ions

Mn²⁺ > Ni²⁺ ≈ Co²⁺ > Mo²⁺ > Re⁴⁺ >Re³⁺ > + > Ir³⁺ >Co³⁺

β can never be greater than unity (β ≤ 1). β = 1 indicates 100% ionic interaction in the metal-ligand. For non-polarisable ligands like F⁻, β is close to unity and for polarisable ligands as I⁻, S²⁻ β is less than unity. The nephelauxetic series of metal ions also depends on the covalent bond forming ability of the metal ions.