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.
a. Based on the spectrum, what is the color of an [Fe(H2O)5SCN]2+ solution?
b. Calculate the crystal field splitting energy, , of [Fe(H2O)5SCN]2+ in kJ>mol.
c. The hexaaquairon(III) complex ion, [Fe(H2O)6]3+, produces a pale violet aqueous solution. Is the crystal field splitting energy, , of [Fe(H2O)6]3+ smaller or larger than the of [Fe(H2O)5SCN]2+?
d. On the basis of your answers to parts b and c, compare the crystal field strengths of water and thiocyanate ligands.
e. 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+?