What explains why many transition metal compounds appear brightly colored?

What explains why many transition metal compounds appear brightly colored?

• A. s-p transitions
• B. d-d transitions
• C. f-f transitions
• D. p-p transitions

Answer: (B)

Light absorption by electrons in the metal’s d-orbitals is what gives transition metal compounds their color. When ligands surround a transition metal ion, they create an electric field that splits the degenerate d orbitals into two sets with different energies (for example, in an octahedral field the lower-energy t2g set and the higher-energy eg set). An electron can be promoted from the lower set to the higher set by absorbing a photon. If the energy gap between these split d-levels falls in the visible part of the spectrum, the compound absorbs some colors and the color you see is the complementary color of what was absorbed, making the compound appear brightly colored. The exact color depends on the metal, its oxidation state, and the ligands, which set the size of the splitting Δ. Some d-d transitions are weak due to selection rules, but vibronic coupling often makes them noticeable, producing vivid colors. In contrast, s-p transitions or p-p transitions are not the typical source of color for these complexes, and f-f transitions (while possible in lanthanides) are usually much weaker and not the general reason for the bright colors of most transition metal compounds.

Light absorption by electrons in the metal’s d-orbitals is what gives transition metal compounds their color. When ligands surround a transition metal ion, they create an electric field that splits the degenerate d orbitals into two sets with different energies (for example, in an octahedral field the lower-energy t2g set and the higher-energy eg set). An electron can be promoted from the lower set to the higher set by absorbing a photon. If the energy gap between these split d-levels falls in the visible part of the spectrum, the compound absorbs some colors and the color you see is the complementary color of what was absorbed, making the compound appear brightly colored. The exact color depends on the metal, its oxidation state, and the ligands, which set the size of the splitting Δ. Some d-d transitions are weak due to selection rules, but vibronic coupling often makes them noticeable, producing vivid colors. In contrast, s-p transitions or p-p transitions are not the typical source of color for these complexes, and f-f transitions (while possible in lanthanides) are usually much weaker and not the general reason for the bright colors of most transition metal compounds.