# How to determine the greatest d orbital splitting?

This question comes specifically from an IB Chemistry HL Paper 1 in May 2018 TZ1, namely question 8.

Which complex has the greatest d orbital splitting?

It gives 4 Complexes $$\ce{[Fe(H_2O)_6]^{2+}}$$, $$\ce{[Fe(H_2O)_6]^{3+}}$$, $$\ce{[Co(H_2O)_6]^{3+}}$$, $$\ce{[Cr(NH_3)_6]^{3+}}$$ and it says that they give the colours green, orange, blue and violet respectively.

Initially I thought that the answer would be $$\ce{[Cr(NH_3)_6]^{3+}}$$ because it gives the highest energy light, being violet. However, the answer is given as $$\ce{[Fe(H_2O)_6]^{3+}}$$, why is this?

• From what I understand, $\ce{[Cr(NH_3)_6]^{3+}}$ would be absorbing the complementary colour of violet being yellow, which is low energy. However, how does this make any sense because $\ce{NH_3}$ is a strong field ligand which should create a large d orbital splitting, therefore being able to absorb higher energy lights? – Anthony P Mar 20 at 2:06