Nickel ion in solution is colored as which of the following?

• A. Red
• B. Blue
• C. Green
• D. Yellow

Answer: (C)

Color of transition-metal ions in solution comes from d-d electronic transitions within the metal center. Nickel(II) in water forms an octahedral complex, [Ni(H2O)6]2+. In this environment the 3d orbitals split into lower-energy t2g and higher-energy eg levels. Transitions between these split levels absorb light in the visible region, and the color we see is the complementary color of the absorbed light. For Ni2+ in aqueous solution, the main absorption band lies in the red part of the spectrum, so red light is absorbed and the solution appears green. Real solutions may shift slightly with concentration or ligand strength, but green is the expected hue for nickel(II) in water.