Aqueous solution of FeCl3 is yellow whereas other metals having the same d5 configuration are pale violet coloured. Explain the origin of the colour [closed]








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The aqueous solution of the other metals with the same d5 configuration give pale violet colour.
Pale colour could be caused by the d-d transitions.
i got to know that Ferric chloride forms a self ionizing complex in aqueous medium as [Fe(H2O)4Cl2]^+[FeCl4]2-  

Now i dont know what type of transitions are taking place in this compound which is giving out the yellow colour.




If, in fact, ferric chloride firms $\ce{FeCl4^-}$, then this ion could indeed be responsible for the different color of the chloride in solution.


Textbooks often describe high-spin $d^5$ complexes as only pale colored because the $d-d$ transition is spin-forbidden (in a nonrelatvistic theory), and what little transition (and thus coloration in white light) cones from (relativistic) spin-orbit coupling. Less attention is paid to the fact that with the most common metal centers that give this configuration, we typically have octahedral complexes, whose center of symmetry or near-symmetry (with nonidrntucal ligands) also renders the $d-d$ transitions forbidden due to lack of an inherent dipole component in the transition (Laporte forbidden). Imperfect symmetry with different ligands, or molecular vibrations, are necessary to get around this barrier.
$\ce{FeCl4^-}$, however, has tetrahedral geometry instead, so no center of symmetry. The spin limitation is still there, but without the Laporte forbidden character we see more color from the spin-orbit coupling. The more intense color of the tetrahedral complex then overwhelms anything the proposed octahedral solution complex may offer.


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