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Q5.17:
What is spectrochemical series? Explain the difference between a weak field ligand and a strong field ligand.

Solution :

Definition: The Spectrochemical Series

The spectrochemical series is an empirically derived arrangement of ligands ordered according to the magnitude of the crystal field splitting energy ($\Delta$) they induce when coordinated to a central metal ion. This ordering is determined primarily by the ligand's ability to cause splitting of the $d$-orbitals of the transition metal, which is observable through the wavelength of light absorbed by the complex [Per Crystal Field Theory (CFT)].

The series is represented as follows:

$I^- < Br^- < S^{2-} < SCN^- < Cl^- < F^- < OH^- < C_2O_4^{2-} < H_2O < NCS^- < EDTA^{4-} < NH_3 < en < CN^- < CO$

Comparative Analysis: Weak Field vs. Strong Field Ligands

The classification of ligands as "weak" or "strong" field depends on their interaction with the $d$-orbitals of the metal. This is characterized by the relative magnitude of the crystal field splitting energy ($\Delta_o$) compared to the electron pairing energy ($P$).

Feature Weak Field Ligand Strong Field Ligand
Field Strength Low ($\Delta_o < P$) High ($\Delta_o > P$)
Effect on Electrons Does not force pairing; high-spin complex. Forces electron pairing; low-spin complex.
Nature of Interaction $\pi$-donor ligands (e.g., halides). $\pi$-acceptor ligands (e.g., $CN^-$, $CO$).
Spectral Shift Absorbs lower energy (longer $\lambda$). Absorbs higher energy (shorter $\lambda$).

Visual Representation of Crystal Field Splitting

Energy Free Metal Ion $e_g$ $t_{2g}$ $\Delta_o$

Theoretical Justification

  • Weak Field Ligands ($\Delta_o < P$): These ligands have a small interaction with the metal's $d$-orbitals. Because the energy required to pair two electrons in the same orbital ($P$) is greater than the energy gap ($\Delta_o$), electrons prefer to occupy the higher energy orbitals ($e_g$) before pairing up in the lower orbitals ($t_{2g}$). This results in a high-spin configuration.
  • Strong Field Ligands ($\Delta_o > P$): These ligands create a large energy separation. Since the cost of pairing electrons is less than the energy required to excite an electron to the higher $e_g$ level, electrons pair up in the $t_{2g}$ orbitals first. This results in a low-spin configuration.

Final Solution: The spectrochemical series is a ranking of ligands based on their crystal field splitting strength. Weak field ligands result in $\Delta_o < P$ (high-spin), whereas strong field ligands result in $\Delta_o > P$ (low-spin).


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