Question

Why do tetrahedral complex ions have a different crystal field diagram than octahedral complex ions? Why are virtually all tetrahedral complex ions “high spin”?

Short answer

In Octahedral complexes, ligands approach along the axes (x, y, z) while in tetrahedral complexes, ligands approach from in between the axes, thus both have different CFT diagrams.

In tetrahedral complexes, the CFSE (Crystal Field Splitting Energy) is relatively low, so, electrons can easily consider jumping to the higher orbitals without going for pairing.

Step-by-step solution

Reason behind different crystal diagrams of tetrahedral and octahedral complex ions.

In tetrahedral complexes, the ligands come in direct contact with${\mathrm{d}}_{\mathrm{xy},}{\mathrm{d}}_{\mathrm{yz},}{\mathrm{d}}_{\mathrm{zx}}$ ligands, which cause them to split to a higher level. These orbitals are located in between the axes, which makes them repel the ligands with lesser energy compared to that octahedral orbital.

In Octahedral complexes, the ligands come in direct contact with${\mathrm{d}}_{{\mathrm{z}}^2}\mathrm{and}{\mathrm{d}}_{{\mathrm{x}}^2-{\mathrm{y}}^2}$ orbitals which are located along the axes and cause them to repel the ligands with higher energy. Thus, octahedral CFSE is always larger than tetrahedral CFSE.

Also, Octahedral complexes are made up of six ligands, while tetrahedral complexes are made up of four ligands leading to different geometries.

Reason behind tetrahedral complexes being “high spin”

As we discussed above, the CFSE for tetrahedral complexes is always less, So the electrons prefer jumping to the higher energy level rather than pairing.