# Which of the following octahedral complexes absorbs the photon with the longest wavelength, λ?

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!

• 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. – porphyrin Nov 18 '20 at 8:53