I have been told that with the help of crystal field theory, we can decide the electronic configuration ($\mathrm{t_{{2g}}}$ $\mathrm{e_g}$ configuration) of the metal ion in a complex. For example, in octahedral complexes, if the ligand is a strong field ligand, pairing of electron in $\mathrm{t_{{2g}}}$ will be favorable, rather than the electron occupying an $\mathrm{e_g}$ orbital. The opposite is true for a weak field ligand.

The spectrochemical series which helps us relatively identify which ligand is a strong field ligand, and which is a weak field one.

I have two questions:

  1. I understand that the ligands towards left side are strong field, and the ligands towards right side are weak field. But let’s say we take a ligand in the middle. Is it strong field or weak field? Which ligand from the left side is the last strong field ligand? Is it even possible to determine that?

  2. Let’s say we have the following compound: $\ce{[CoCl2(en)2]Cl}$. Now, as $\ce{Cl}$ is a weak field ligand and en is a strong field ligand, how exactly do we decide the electronic configuration? Is there any general method which considers cases like this?

Please bear in mind that I am a high school student, and I might not be able to understand any advanced theory involved.


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