Question 22.1: The first modern synthetic color, Prussian blue, was made in...
The first modern synthetic color, Prussian blue, was made in 1704 in Berlin, Germany. It was heavily used in the 19th-century Japanese woodblock print The Great Wave off Kanagawa (Figure 22.4), and it continues to be popular with artists worldwide. The formula for the insoluble pigment is Fe_{4}[Fe(CN)_{6}]_{3}.
a. What is the formula of the complex ion in this compound and what is its charge if the counterion is Fe_{3}^{+}?
b. What is the oxidation state of iron in the complex ion?
Collect, Organize, and Analyze We have the formula of a coordination compound and are asked to identify the complex ion and the counterion. The complex ion is contained in brackets, the four Fe_{3}^{+} counterions are outside the brackets, and the compound must be electrically neutral.
Solve
a. The combined charge of four Fe_{3}^{+} is 12+. To achieve electrical neutrality, the combined charge of the three complex anions must be 12–. Therefore the charge of each anion is (12–)/3 = 4– , and the formula for the anion is [Fe(CN)_{6}]^{4-} .
b. Six cyanide ligands, CN^{-} , occupy the inner coordination sphere for a total charge of 6–. Therefore the charge on Fe must be 2+ for the charge on the complex ion to equal 4–.
Think About It Complex ions may have positive or negative charges. Correspondingly, counterions will have charges opposite to those of the complex ions to ensure electrical neutrality.