"This blue is very deep."
Kana gazed at a copper complex solution.
"Copper sulfate solution," Milia answered. "Cu²⁺ ions surrounded by water molecules."
"Why is it blue?"
Rei opened his notebook. "Because d-electron energy levels split."
"Split?"
"Transition metal ions have electrons in d-orbitals. Normally, the five d-orbitals have nearly the same energy."
Milia supplemented, "But when ligands approach, the situation changes."
"Ligands... you mean water molecules?"
"Right. The lone electron pairs of water molecules coordinate to the metal ion."
Rei drew a diagram on the whiteboard. "Six water molecules arrange octahedrally around the copper ion."
"Beautiful arrangement."
"At this time, the five d-orbitals split into two groups."
Kana asked curiously, "Why do they split?"
"Because the distance to ligands differs," Rei explained. "The dz² and dx²-y² orbitals point toward the ligands."
"So repulsion is stronger," Milia continued.
"Accordingly, their energy becomes higher. Meanwhile, dxy, dyz, dzx orbitals point between ligands."
"Repulsion is weaker, so energy is lower," Kana understood.
"Right. This splitting is called Δo, octahedral splitting."
Toma entered the clubroom. "Making blue liquid?"
"Learning about metal complex colors," Kana answered.
Toma showed interest. "Then why is copper blue and iron reddish-brown?"
Rei explained, "Because Δo magnitude differs. And that value determines the wavelength of light absorbed."
"Absorbed?"
"D-electrons are excited from lower to higher energy levels. At that time, specific wavelengths of light are absorbed."
Milia supplemented, "For Cu²⁺, red light is absorbed. So we see the complementary color, blue."
Kana was surprised. "The complement of the absorbed color is what we see."
"Exactly," Rei nodded.
Toma took out another reagent bottle. "Then this green nickel complex?"
"It absorbs both red and blue," Rei answered. "So green remains."
"Does the color change if you change ligands?" Kana asked.
"Good question," Milia smiled. "Δo magnitude changes with ligand type."
Rei wrote the ligand series. "I⁻ < Br⁻ < Cl⁻ < F⁻ < H₂O < NH₃ < en < CN⁻."
"Moving right, Δo increases."
"Meaning the wavelength of absorbed light changes," Kana understood.
Toma started an experiment. "Let's add ammonia to copper ions."
The solution color changed. From light blue to deep blue-purple.
"Wow!" Kana exclaimed.
"Ammonia is a stronger ligand than water," Rei explained.
"So Δo increased and the absorption wavelength changed."
Milia said quietly, "Color is a mirror reflecting electron energy states."
"Beautiful," Kana murmured.
"And," Rei continued, "metal complexes in living organisms work on the same principle."
"Hemoglobin?"
"Right. Iron-porphyrin complex. When oxygen coordinates, the color changes."
"Arterial blood is red, venous blood is dark because..."
"The coordination state changes with oxygen presence," Milia nodded.
Toma was impressed. "Chemistry is all around us."
"Visual understanding is what makes metal complexes interesting," Rei said.
Kana wrote in her notebook. "Color tells the molecular electronic state."
"And," Milia added, "that color supports life phenomena."
Sunset streamed through the lab window. The blue of the solution shone more deeply.
"Next, let's try with different metals," Kana's eyes sparkled.
"Cobalt, chromium, manganese... all sorts of colors," Toma laughed.
"The rainbow of transition metals," Rei said quietly. "The beauty of chemistry is there."