Explain why [CoCl₄]^2- (blue) and [Co(H₂O)₆]²⁺ (pink) have different colors. Which complex has its absorption bands at longer wavelengths?

Key Concepts

Crystal Field Theory

Crystal Field Theory explains how the arrangement of ligands around a central metal ion affects the energy levels of the d-orbitals. In octahedral complexes, such as [Co(H2O)6]2+, the d-orbitals split into two energy levels due to ligand interactions. This splitting influences the color observed, as different wavelengths of light are absorbed to promote electrons to higher energy levels.

Ligand Field Strength

Ligand field strength refers to the ability of a ligand to influence the energy of the d-orbitals in a metal complex. Strong field ligands, like water in [Co(H2O)6]2+, cause greater splitting of the d-orbitals, leading to higher energy transitions and different colors. In contrast, weaker field ligands, like chloride in [CoCl4]2-, result in smaller splitting and different absorption characteristics.

Color and Absorption

The color of a complex ion is determined by the wavelengths of light it absorbs. When a complex absorbs light, the remaining light is what we perceive as its color. The complex that absorbs light at longer wavelengths will appear in the complementary color; thus, [CoCl4]2- (blue) absorbs light in the red region, while [Co(H2O)6]2+ (pink) absorbs light at shorter wavelengths, indicating that [CoCl4]2- has its absorption bands at longer wavelengths.