Nephelauxetic effect

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. 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 reduction of B from its free ion value is normally reported in terms of the nephelauxetic parameter β:

$${\displaystyle \beta ={\frac {B_{\text{complex}}}{B_{\text{free ion}}}}}$$ 

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]

F⁻ < H₂O < NH₃ < en < [NCS - N]⁻ < Cl⁻ < [CN]⁻ < Br⁻ < N₃⁻ < I⁻

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. The ordering roughly reflects the ability of the ligands to form good covalent bonds with metals - those that have a small effect are at the start of the series, whereas those that have a large effect are at the end of the series.

The nephelauxetic effect does not only depend upon the ligand type, but also upon the central metal ion. These too can be arranged in order of increasing nephelauxetic effect as follows:^{[citation needed]}

Mn(II) < Ni(II) ≈ Co(II) < Mo(II) < Re(IV) < Fe(III) < Ir(III) < Co(III) < Mn(IV)

• Spectrochemical series
• Complex (chemistry)

• Housecroft C.E. and Sharpe A.G., Inorganic Chemistry, 2nd Edition, England, Pearson Education Limited, 2005. p. 578.
• Shriver D.F and Atkins P.W, Inorganic Chemistry, 4th Edition, England, Oxford University Press, 2006. p. 483.