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Electron repulsion is the repulsive electrostatic interaction between electrons. In a metal complex it affects pairing costs and many-electron transition energies, so a measured absorption maximum is not always identical to a one-electron orbital splitting.

The relevant spectrochemical series order is O-bound nitrito $<$ ammonia $<$ N-bound nitro. A pi-donor ligand tends to raise the metal orbitals derived from $t_{2g}$, reducing the octahedral splitting; a pi-acceptor ligand stabilizes those orbitals, increasing the splitting. Since photon energy is $E=hc/\lambda$, the stronger-field complex absorbs at the shorter wavelength. ThusThe corresponding spectroscopic wavenumbers are approximately $21834$, $21097$ and $20576\mathrm{c}{\mathrm{m}}^{-1}$. These are the observed band energies, not three exact independent determinations of ${\Delta }_o$: low-spin $d^6$ transition energies also contain electron repulsion contributions, and replacing one ligand lowers the ideal octahedral symmetry. The relative assignment is nevertheless robust.