Ionic interaction in crystal field theory

How correct is the assumption of ionic interaction between the ligands and the central metal atom in cft when many ligands do not even possess charge over them?


Besides, I also referred to the conditions required for formation of ionic bond. They are mostly formed when electronegativity difference between the two atoms is around 2.0 on the Pauling scale. The EN difference between ligands and metals is less than that mentioned above in most of the cases. So on what assumptions does the idea of ionic interaction hold?




Crystal field theory does not really assume ionic interactions between the ligand and the central metal. It assumes that the ligands can be replaced in a thought experiment by point-shaped negatively charged particles that approach the central metal from certain directions causing the orbital energies to change.


The idea of seeing them as negatively charged has its merits. All ligands (formally) ligate through the action of a lone pair, whether this lone pair contributes to a charge, a formal charge or not. Note that positively charged ligands can occur; the most famous being $\ce{NO+}$. The electron pair that ligates will, however, *always* carry some local negative charge just because electrons are negatively charged particles.


This should not really be part of the question but I’ll mention and answer it anyway. For one, you seem to be assuming that ionic and covalent bonds are two different things. That is not true. All bonds between different elements can be understood to have a partially covalent and a partially ionic character; the main difference is which character is stronger. And the $2.0$ difference on the Pauling scale is part of the Pirate Code: more of a rough guideline than an actual rule. You can find counterexamples in both directions: predominantly ionic bonds with less and predominantly covalent bonds with more electronegativity difference. However, as I mentioned this is not relevant to crystal field theory because you should not be assuming an ionic bond between the ligand and the metal. 


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