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

Verified step by step guidance

1. 1. The color of a complex ion depends on the difference in energy between the d orbitals. This difference is determined by the ligands that are attached to the metal ion. In this case, we have two different ligands: Cl- and H2O.
2. 2. The ligand Cl- is a stronger field ligand than H2O. This means that it causes a larger splitting of the d orbitals in the Co ion. The larger the splitting, the higher the energy of the light that is absorbed (and thus, the shorter the wavelength).
3. 3. Therefore, the [CoCl4]2- complex absorbs light of higher energy (shorter wavelength) than the [Co(H2O)6]2+ complex. This is why they have different colors: [CoCl4]2- appears blue because it absorbs light in the orange-red part of the spectrum, while [Co(H2O)6]2+ appears pink because it absorbs light in the green part of the spectrum.
4. 4. The complex that has its absorption bands at longer wavelengths is the one that absorbs lower energy light. In this case, that would be the [Co(H2O)6]2+ complex.
5. 5. Remember that the color we see is the color that is not absorbed by the complex. So, a complex that absorbs green light will appear red, and a complex that absorbs orange-red light will appear blue.

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.