Nephelauxetic effect

The nephelauxetic effect is a term used in the inorganic chemistry of transition metals.^[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 name "nephelauxetic" comes from the Greek for cloud-expanding. The presence of this effect brings out the disadvantages of crystal field theory, as this accounts for somewhat covalent character in the metal-ligand interaction.

Racah parameter

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

β= B(complex) / B (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.

Ligands

The list shown below list commons ligands (with 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 and are at the start of the series, whereas those that have a large effect are at the end of the series.

Central metal ion

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:

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

See also

• Spectrochemical series
• Complex (chemistry)

References

1. Tchougréeff, Andrei L.; Dronskowski, Richard (2009). "Nephelauxetic effect revisited". International Journal of Quantum Chemistry 109 (11): 2606–2621. doi:10.1002/qua.21989. ISSN 0020-7608.
2. Juranic, Nenad. (1988). "Nephelauxetic effect in paramagnetic shielding of transition metal nuclei in octahedral d6 complexes". Journal of the American Chemical Society 110 (25): 8341–8343. doi:10.1021/ja00233a010. ISSN 0002-7863.
3. Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. p. 578. ISBN 978-0130399137.

Further reading

• 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.