"Beautiful..."
Kana gazed at the test tube containing a vivid blue solution.
"It's copper sulfate solution," Toma explained.
"Why is it blue?"
Rei approached. "Because copper ions form complexes with water molecules."
"Complex?"
"A structure where ligand molecules bond to a central metal ion."
Milia took out another test tube. The contents were green. "Nickel chloride."
"Nickel is green."
"More precisely, hexaaquanickel(II) ion is green," Rei corrected.
Toma excitedly brought out another example. "Then what about this?" Vivid purple.
"Potassium permanganate," Kana answered.
"Correct. Manganese(VII) ion shows purple."
"But why do metal ions have different colors?" Kana asked the essence.
Rei began drawing a diagram. "Transition metals have incompletely filled d orbitals."
"d orbitals?"
"Regions where electrons can exist. In transition metals, d orbitals are partially filled."
Milia supplemented. "When ligands approach, d orbital energies split."
"Ligand field splitting," Rei stated the term.
Toma assembled a 3D model. "Six ligands arrange in an octahedral shape."
"Then the five d orbitals divide into two energy groups."
Kana tried to draw in her notebook but couldn't do it well. "Difficult..."
"Simply put," Rei explained gently. "d orbitals in the direction of ligands have higher energy. Because of repulsion."
"d orbitals in directions without ligands remain at relatively lower energy."
Milia drew a diagram showing it. A neatly organized orbital diagram.
"This energy difference determines the color of light absorbed," Rei continued.
"For copper ions, red light is absorbed. So the complementary color blue is seen."
Toma took out a color wheel. "Absorbed and seen colors are complementary."
"Absorb red see blue, absorb green see purple."
Kana began to understand. "So if you change the ligand, the color changes too?"
"Sharp," Rei acknowledged.
Milia started an experiment. Adding ammonia to a copper ion solution.
The color changed. From light blue to deep blue.
"Tetraamminecopper(II) ion," Rei explained. "Ligands changed from water to ammonia."
"When ligands change, d orbital splitting width changes too."
"So the wavelength of absorbed light changes, and color changes."
Toma did another experiment. Adding concentrated hydrochloric acid to nickel solution.
Green changed to blue.
"Changed from six-coordinate to four-coordinate. When structure changes, splitting width changes."
Kana's face showed emotion. "Complexes are so sensitive."
"Yes. That's why complex color changes are used in analytical chemistry."
Milia said quietly. "Hemoglobin is also a complex."
"Hemoglobin?"
"A complex with iron ion at the center. When oxygen coordinates, the color changes."
"Arterial blood is bright red and venous blood is dark red because of that," Rei supplemented.
"Blood color is also due to ligand differences..." Kana murmured.
Toma opened a biology textbook. "Chlorophyll is also a complex. The center is magnesium."
"Vitamin B12 too. The center is cobalt."
Milia said finally. "Complexes create life's colors."
Rei looked out the window. "Plant green, blood red, sky blue. All from light absorption by electron excitation."
"What shines at the center of complexes are transition metal electrons."
Kana wrote largely in her notebook. "Color is a window showing electron energy differences."
Sunset illuminates the laboratory. Complexes continue their dialogue of light and electrons today too.