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This is a rather specific question, but why doesn't the field splitting parameter increase steadily along the period for 3rd-row transition metals, as one would expect due to better an energy match between ligands and the metal as $Z_\mathrm{eff}$ increases?

The series is (in the order of increasing $\Delta$: $\ce{Mn^2+}, \ce{Ni^2+}, \ce{Co^2+}, \ce{Fe^2+}, \ce{V^2+}$ etc. Source: Spectrochemical series – Wikipedia, confirmed by Shriver and Atkins.

I would guess that this is to do with $\mathrm{3d}$ orbitals getting too contracted on the right of the period and, and hence, having a poorer size match, but I couldn't find this explanation mentioned anywhere.

Why doesn't the field splitting parameter increase steadily along the period for 3rd-row transition metals, as one would expect due to a better energy match between ligands and the metal as $Z_\mathrm{eff}$ increases?

The series is (in the order of increasing $\Delta$: $\ce{Mn^2+}, \ce{Ni^2+}, \ce{Co^2+}, \ce{Fe^2+}, \ce{V^2+}$ etc. Source: Spectrochemical series – Wikipedia, confirmed by Shriver and Atkins.

I would guess that this is to do with $\mathrm{3d}$ orbitals getting too contracted on the right of the period and, and hence, having a poorer size match, but I couldn't find this explanation mentioned anywhere.

This is a rather specific question, but why doesn't the field splitting parameter increase steadily along the period for 3rd row transition metals, as one would expect due to better a energy match between ligands and the metal as $Z_\mathrm{eff}$ increases?

The series is (in the order of increasing $\Delta$: $\ce{Mn^2+}, \ce{Ni^2+}, \ce{Co^2+}, \ce{Fe^2+}, \ce{V^2+}$ etc. Source: Spectrochemical series – Wikipedia, confirmed by Shriver and Atkins.

I would guess that that this is to do with $\mathrm{3d}$ orbitals getting too contracted on the right of the period and, and hence, having a poorer size match, but I couldn't find this explanation mentioned anywhere.

This is a rather specific question, but why doesn't the field splitting parameter increase steadily along the period for 3rd-row transition metals, as one would expect due to better an energy match between ligands and the metal as $Z_\mathrm{eff}$ increases?

The series is (in the order of increasing $\Delta$: $\ce{Mn^2+}, \ce{Ni^2+}, \ce{Co^2+}, \ce{Fe^2+}, \ce{V^2+}$ etc. Source: Spectrochemical series – Wikipedia, confirmed by Shriver and Atkins.

I would guess that this is to do with $\mathrm{3d}$ orbitals getting too contracted on the right of the period and, and hence, having a poorer size match, but I couldn't find this explanation mentioned anywhere.

This is a rather specific question, but why doesn't the field splitting parameter increase steadily along the period for 3rd row transition metals, as one would expect due to better a energy match between ligands and the metal as $Z_{eff}$ increases?

The series is (in the order of increasing $\Delta$: $Mn^{2+}, Ni^{2+}, Co^{2+}, Fe^{2+}, V^{2+}$ etc. Source: Spectrochemical series – Wikipedia, confirmed by Shriver and Atkins.

I would guess that that this is to do with $3d$ orbitals getting too contracted on the right of the period and, and hence, having a poorer size match, but I couldn't find this explanation mentioned anywhere.

This is a rather specific question, but why doesn't the field splitting parameter increase steadily along the period for 3rd row transition metals, as one would expect due to better a energy match between ligands and the metal as $Z_\mathrm{eff}$ increases?

The series is (in the order of increasing $\Delta$: $\ce{Mn^2+}, \ce{Ni^2+}, \ce{Co^2+}, \ce{Fe^2+}, \ce{V^2+}$ etc. Source: Spectrochemical series – Wikipedia, confirmed by Shriver and Atkins.

I would guess that that this is to do with $\mathrm{3d}$ orbitals getting too contracted on the right of the period and, and hence, having a poorer size match, but I couldn't find this explanation mentioned anywhere.