Question
What is spectrochemical series? Explain the difference between a weak field ligand and a strong field ligand.
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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,
$\ce{I– < Br^– < SCN^– < Cl^{– }< S^{2–} < F ^– < OH^– < C_2O_4^{2–} < H_2O < NCS^{– }< edta^{4–} < NH_3 < en < CN^– < CO}$
The ligand present on the $\ce{R.H.S}$ of the series are strong field ligand while $\ce{L.H.S}$ are weak field ligand. Also, strong field ligand cause higher splitting in the $d-$ orbitals than weak field ligand.
$\ce{I– < Br^– < SCN^– < Cl^{– }< S^{2–} < F ^– < OH^– < C_2O_4^{2–} < H_2O < NCS^{– }< edta^{4–} < NH_3 < en < CN^– < CO}$
The ligand present on the $\ce{R.H.S}$ of the series are strong field ligand while $\ce{L.H.S}$ 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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