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When I learned about crystal field theory I had fallen on a paradox. When K4[Fe(CN)6] is formed then three d orbitals are occupied by electrons of Fe2+ pairly and the rest two d orbitals are occupied by electrons of lone pair electron of CN-. So when visible light falls on it, the d orbitals are splitted into two sets which are t2g and eg and the electrons of eg orbitals jump into t2g orbitals by absorbing a portion of this light. But my question is that how the electrons of eg orbitals jump into t2g orbitals when the d orbitas are already occupied by the electrons of Fe2+ and ligand during bonding.

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  • $\begingroup$ They are jumping into an antibonding orbital. $\endgroup$ – Blaise Jul 24 '20 at 16:20
  • $\begingroup$ In crystal field theory, the electrons of the ligands are not distributed among the Fe central atom. They stay outside. So the electrons from CN are not on d orbitals. The six d electrons of the ion $\ce{Fe^{2+}}$ are all paired on the three t2g levels, which are lower than the two eg levels. This splitting happens without any light. When light is adsorbed, one of these six electrons is excited and goes to one of the eg levels, which are unoccupied. Where is the problem ? $\endgroup$ – Maurice Jul 24 '20 at 19:33

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