Final answer:
Transition metal compounds exhibit bright colors due to d-d transitions, where light absorption causes electron excitation within d-orbitals. The nature of ligands affects the color by causing different energy splits in d orbitals, with strong-field ligands leading to yellow to red colors, and weak-field ligands to blue or green hues.
Step-by-step explanation:
Transition metal compounds generally exhibit bright colors because of the d-d transitions of electrons within their d-orbitals. When visible light passes through these compounds, certain wavelengths are absorbed as electrons are excited from lower-energy d orbitals to higher-energy d orbitals. The remaining light, which is not absorbed, imparts the distinctive color we see. For example, compounds with strong-field ligands absorb higher-energy violet or blue light, appearing yellow, orange, or red. In contrast, complexes with weak-field ligands absorb lower-energy yellow, orange, or red light, and therefore often appear blue-green, blue, or indigo.
This phenomenon is due to the specific energy levels required for a photon to effect the d-d transition, which depends on the magnitude of splitting in the d orbital energies (Aoct or A₁). Minerals such as malachite (green), azurite (blue), and proustite (red) are vivid examples of transition metal complexes exhibiting such colors.