Q23.89P - Chemistry: Molecular Nature Of Matter And Change

Question

Why are there both high-spin and low-spin octahedral complexes but only high-spin tetrahedral complexes?

Step-by-Step Solution

Answer

There are both high-spin and low-spin octahedral complexes because ligands can approach both directly and between d orbitals and in tetrahedral complexes none of the d orbitals is directly pointed to approaching ligands.

Step 1: Octahedral complexes with low-spin and high-spin

Octahedral complexes:

- When strong-field ligands approach central metal ion, they are directly pointed to higher energy $\left({\mathrm{e}}_g\right)$ d orbitals, they cause big splitting energy  and the low-spin complexes are created.

 

- When weak-field ligands approach central metal ion, they are pointed between the lower energy $\left({\mathrm{t}}_{2g}\right)$ d orbitals, they cause small splitting energy $\left(\Delta <{\mathrm{E}}_{\mathrm{pairing}}\right)$ and the high-spin complexes are created.

Step 2: Tetrahedral complexes with high spin

Tetrahedral complexes:

- When ligands approach the central metal ion, none of the d orbitals is directly pointed to them and the splitting energy is smaller than in octahedral complexes. There are only high-spin tetrahedral complexes because $∆$ is always smaller than E pairing (electrons fill $t_{2g} \mathrm{d}$ orbitals).