Iron(II) ions react with phenanthroline (C₁₂H₈N₂) to form a complex ion with the general formula Fe_x(C₁₂H₈N₂)_y ^2x+. A student uses the method of continuous variation to determine the number of phenanthroline molecules bound to each iron ion (i.e., the values of x and y in the formula). The complex ion is strongly colored, so its concentration can be monitored using a spectrometer. Working with 2.4 × 10^-3 M solutions of both Fe³⁺and C₁₂H₈N₂, the data shown in the graph below were obtained. What is the correct formula for the complex ion?

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Concentration of complex ion 0 10 T 20 30 T 40 50 60 Volume % of Fe²+ 70 80 90 100