Question
What is spectrochemical series? Explain the difference between a weak field ligand and a strong field ligand.
✓
Answer
Spectro-chemical series is a series in which the ligands have been arranged in order of increasing magnitude of splitting they produce. The order is,
$I– < Br^– < SCN^– < Cl^–< S^{2–} < F ^– < OH^– < C_2O_4^{2–} < H_2O < NCS^–< edta^{4–} < NH_3 < en < CN^– < CO$
The ligand present on the R.H.S of the series are strong field ligand while L.H.S are weak field ligand. Also, strong field ligand cause higher splitting in the d- orbitals than weak field ligand.
$I– < Br^– < SCN^– < Cl^–< S^{2–} < F ^– < OH^– < C_2O_4^{2–} < H_2O < NCS^–< edta^{4–} < NH_3 < en < CN^– < CO$
The ligand present on the R.H.S of the series are strong field ligand while L.H.S are weak field ligand. Also, strong field ligand cause higher splitting in the d- orbitals than weak field ligand.
| Weak field ligand | Strong field ligand | |
| 1. | They are formed when the crystal field stabilisation energy $(\triangle _0)$ in octahedral complexes is less than the energy required for an electron pairing in a single orbital (p). | They are formed when the crystal field stabilisation energy $(\triangle _0)$ is greater than the p. |
| 2. | They are also called high spin complexes. | They are called low spin complexes. |
| 3. | They are mostly paramagnetic in nature complex. | They are mostly diamagnetic or less paramagnetic than weak field. |
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