Some complexes absorb in more than one area of the visible spectrum, but you can

Question

Some complexes absorb in more than one area of the visible spectrum, but you can still determine the color, What will this complex be? The pictures below are of ferrous ammonium sulfate (left), which contains the hexaaquo ferrous ion[Fe(H_2O)_4]^2+, and potassium ferrocyanide (right), which contains the ion [Fe(CN)_4]^4-. Both complexes contain the contain. the Fe^2+ cation. color The Cr^3+ cation also forms complex ions with water and cyanide. [Cr(H_2O)_6]^3+ is a violet color, while [Cr(CN)_6]^-3- is yellow color. From these observations, what can you say about the energies of the transitions for the complexes with the cyanide versus the transitions in the complexes with the water ligands?

Explanation / Answer

There are two comples of Cr as follows:

[Cr (H2O)6]3+   Violet

[Cr(CN)6]3-       Yellow

The oxidation number of Cr in both complex = III

But in both complex there are two different type of ligand H2O, and CN which will changes to energy of transition .

The absorption bands of these complexes move to lower energies in the order of the spectro-chemical series for the ligands:

NO > CO PF CN > NO > NH > HO > OH > F > S² > Cl > Br > I

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