What is the expected color of [Co(NH3)6]³+?

The expected color of the complex ion [Co(NH3)6]³+ is blue. This is due to the presence of cobalt in a +3 oxidation state and the coordination of ammonia ligands, which significantly influence the color of the complex through electronic transitions.

Cobalt(III) complexes often exhibit colors that arise from d-d transitions in the visible region of the spectrum. The specific color of a complex is determined by its ligand field strength, which affects the splitting of the d-orbitals. In the case of ammonia as a ligand, it is considered a moderately strong field ligand, resulting in substantial splitting of the d-orbitals and allowing the absorption of certain wavelengths of light.

For [Co(NH3)6]³+, it absorbs light primarily in the red region of the spectrum, leading to the complementary color being blue. This common phenomenon of complementary colors helps to predict the observed color of the complex.

Overall, the characteristic blue color of [Co(NH3)6]³+ highlights the role of ligand interactions and electronic transitions in determining the appearance of transition metal complexes.