Question

Why are low spin tetrahedral complexes rarely formed?

Hint:

Tetrahedral splitting is small, so pairing usually does not occur. Hence tetrahedral complexes are generally high spin.

Answer 1

Step 1: Crystal field splitting in tetrahedral geometry.
In a tetrahedral field: \[\Delta_t = \frac{4}{9}\Delta_o\] So $\Delta_t$ is only about 44% of $\Delta_o$ for the same metal-ligand system.
Step 2: Comparison with pairing energy.
Since $\Delta_t$ is very small, it rarely exceeds the pairing energy $P$. Electrons therefore prefer to remain unpaired, giving high spin configurations.
Step 3: Conclusion.
\[ \boxed{\Delta_t = \frac{4}{9}\Delta_o \ll P \Rightarrow \text{tetrahedral complexes are nearly always high spin.}} \]