Spectrochemical Series

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Among the ligands iodide, fluoride, water and cyanide, which one produces the largest crystal field splitting around a given metal ion?

RankGuru · Accepted Answer

The spectrochemical series arranges ligands in order of their ability to split the metal d orbitals, measured by the splitting parameter. Strong-field ligands cause large splitting, while weak-field ligands cause small splitting. The accepted order for the listed species is I^- < F^- < H2O < CN^-, so cyanide produces the largest splitting. Cyanide is a strong-field ligand partly because it can accept electron density through pi back-bonding, strengthening the metal-ligand interaction and widening the gap between t_{2g} and e_g sets. Iodide is a large, polarisable, weak-field donor and gives the smallest splitting. Fluoride and water are intermediate but both weaker than cyanide. A large splitting favours low-spin configurations and is the reason cyanide complexes are often diamagnetic. This ordering is exactly the spectrochemical trend taught in NCERT and used to predict colour and magnetism. Plausibility check: cyanide's reputation for forming stable low-spin complexes such as hexacyanoferrate is fully consistent with it lying at the strong-field end of the series.

Spectrochemical Series

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