After learning about crystal field theory, my main question was due to the arrangement of the molecules in the spectrochemical series. I can understand that the larger anions are placed towards the lower end since they do not cause high splitting in the d orbitals, but some of the arrangements of the molecules confuse me. My reasoning is largely due to the size of the molecules. For ions like I-, they have a large ionic radius and cannot pair well with the central metal well. This results in a low Δo. I can also see that CO is smaller and since it is not charged, the lone pair electrons on the oxygen atom can yield high splitting without being repelled by the outer electrons too much. My primary question is why OH- is not towards the end with higher Δo. Any clarification or corrections to what I have posted would be greatly appreciated. Also, any links or another explanation for the arrangement of the molecules on the spectrochemical series would be amazing! Thanks!