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I wonder which of the following absorbs photons with the longest wavelength, $\ce{[Cr(CN)6]^3-}$, $\ce{[Cr(SCN)6]^3-}$ and $\ce{[Cr(H2O)6]^3+}$.

I know the one to absorb the highest wavelength should also have the highest spin. Since all of these are coordination isomers, I think the strong-field ligands should have higher energies (making the answer $\ce{[Cr(CN)6]^3-}$).

I hope someone can clarify the relation between ligand field strength and energy. Any help is much appreciated!

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    $\begingroup$ Look up the spectrochemical series, although empirical this is good at predicting the d orbital splitting and so absorption band frequencies for metals in 'normal' oxidation states. $\endgroup$ – porphyrin Nov 18 '20 at 8:53
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Since CN- is a strong field /pi-acceptor ligand, therefore, it will create larger splitting. So for transition higher energy will be needed. It means that in case of Complexes with CN- higher energied photons i.e. photons with shorter wave length will be absorbed.

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