Question

Explain Crystal Field Theory (CFT) in coordination compounds and write its limitations.

Hint:

Remember: Octahedral → \(t_{2g}\) lower, \(e_g\) higher; Tetrahedral → reverse splitting.

Answer 1

Step 1: Introduction.
Crystal Field Theory (CFT) explains the bonding in coordination compounds in terms of the effect of the electric field produced by ligands on the \(d\)-orbitals of the central metal ion. Step 2: Splitting of d-orbitals.
In a free metal ion, all five \(d\)-orbitals are degenerate (equal energy).
When ligands approach the central metal ion: - In an octahedral field, the \(d\)-orbitals split into two sets: \[ t_{2g} (d_{xy}, d_{xz}, d_{yz}) \quad \text{(lower energy)} \] \[ e_g (d_{z^2}, d_{x^2-y^2}) \quad \text{(higher energy)} \] - The energy difference is called the
crystal field splitting energy (\(\Delta\)). Step 3: Consequences.
\begin{enumerate} \item Explains colour of complexes due to \(d \rightarrow d\) transitions. \item Explains magnetic properties (high spin or low spin) based on pairing of electrons and magnitude of \(\Delta\). \end{enumerate} Step 4: Limitations of CFT.
\begin{enumerate} \item It treats metal-ligand bonds as purely ionic and ignores covalent character. \item Cannot explain the spectra of some complexes accurately. \item Does not account for ligand-metal orbital overlap or back bonding. \end{enumerate} Conclusion:
Crystal Field Theory successfully explains splitting of \(d\)-orbitals, colour, and magnetic properties of coordination compounds, but fails to explain covalent interactions and detailed spectra.