I know how the crystal field stabilization energy is calculated, but how does it reflect stability? CFSE measures the energy relative to the barycentre, not the initial energy of the $d$ orbitals. For example,a $d^5$ configuration in a weak ligand field would have zero CFSE, but that certainly does not mean that the complex is unstable.
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$\begingroup$ You nailed it, and there is not much left to add. $\endgroup$– JanCommented Sep 27, 2015 at 0:21
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$\begingroup$ @Jan So it doesn't relate to stability? $\endgroup$– Tony StarkCommented Jan 12, 2021 at 13:05
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Lets start from the beginning Firstly the metal orbitals (eg- like 3d) are degenerate, now as soon as the ligand surrounds the metal this degeneracy between the orbitals is lost (continuing the same example - the energy of five 3d orbitals is no longer the same) The orbitals of the subshell are repelled by the ligands depending on their orientation relative to that of ligands, and they split into few sets
Now, the orbitals are generally grouped into 2 sets (taking a very classic high school example of splitting in case of octahedrally surrounded central metal atom by ligands) One is between the axes and other along the axes and so now we assume the degeneracy between these 2 sets of orbitals of a subshell, And the important part is that the cfse that is calculated,is with respect to the energy of the whole original set of orbitals (ie wrt barycentre) ( Eg-when the whole 3d was degenerate and ligand field was strong to increase the energy of complete set of orbitals but not enough to split them, as the ligands come closer the splitting happens, that is the reason why in standard textbooks we generally have 3 diagram representation of this phenomenon, what i am talking is about the 2nd to 3rd diagram transition)
And now finally how is this splitting energy is related to stability
Lets say we measure the energies wrt barycentre , so now the orbitals of higher energy will have positive energy and of lower will have negetive energy, and because of the splitting most of the times this relative energy (whose fancy name is cfse) comes out to be negetive, that is the energy of the system has decreased because of splitting and hence more stable so its not really wrong to call crystal field splitting energy as crystal field stabilisation energy
Though there also many cases where cfse comes out to be positive,but then again there are other interesting and intriguing explanations for them!
