Chapter 22, Problem 86E

Many aqueous solutions of complex ions display brilliant colors that depend on the identities of the metal ion and ligand(s). Some ligands bind selectively to certain metal ions and produce a complex ion with characteristic colors. These distinctive complex ions serve as qualitative indicators of the presence of particular metal ions. For example, Fe³+is identified by the rapid formation of the intensely colored pentaaquathiocyanatoiron(III) complex ion. [Fe(H₂O)₅SCN]2⁺, when thiocyanate, SCN^-,is added to a solution containing hexaaquairon(III), [Fe(H₂O)₆]3+, according to the balanced chemical equation shown here:

Examine the absorption spectrum of an aqueous solution of [Fe(H₂O)₅SCN]2+ shown here and answer the questions.

Absorption spectrum of [Fe(H₂O)₅SCN]2+

1. Based on the spectrum, what is the color of an [Fe(H₂O)₅SCN]2+ solution?

Calculate the crystal field splitting energy, Δ, of [Fe(H₂O)₅SCN]2+ in kJ/mol

The hexaaquairon(III) complex ion,

[Fe(H₂O)₆]3⁺, Produces a pale violet aqueous solution. Is the crystal field splitting energy, Δ, of [Fe(H₂O)₆]3+ smaller or larger than the Δ of [Fe(H₂O)₅SCN]2⁺?

On the basis of your answer to parts b and c, compare the crystal field strengths of water and thiocyanate ligands.

The complex ion hexacyanoferrate(III).

[Fe(CN)₆]3^-, is red in aqueous solution. What can you conclude about the relative crystal field splitting energies of [Fe(CN)₆]3- and [Fe(H₂O)₅SCN]2⁺?