- The oxidation state tells us how strongly the metal pulls on the surrounding ligands.
- If the metal has a higher oxidation state, it pulls the ligands closer. This makes the repulsion stronger, so the gap between the two groups of dd-orbitals becomes larger.
- If the metal has a lower oxidation state, it does not pull the ligands as strongly. The ligands stay farther away, so the repulsion is weaker and the gap between the orbitals is smaller.
In short: Higher oxidation state = bigger splitting; Lower oxidation state = smaller splitting.