Solutions of \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+},\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}(\) both octahedral \()\) and \(\left[\mathrm{CoCl}_{4}\right]^{2-}\) (tetrahedral) are colored. One is pink, one is blue, and one is yellow. Based on the spectrochemical series and remembering that the energy splitting in tetrahedral complexes is normally much less that that in octahedral ones, assign a color to each complex.

The spectrochemical series ranks the ligands in order of their ability to cause the energy splitting in the metal orbitals. The series can be given as: I^− < Br^− < Cl^− < F^− < OH^− < H_2O < NH_3 < en < NO_2^− < CN^− Ammonia (NH_3) and water (H_2O) are stronger ligands than chloride (Cl^−) in this series. Stronger ligands produce higher energy splitting.

Typically, the energy splitting in octahedral complexes (Δ_oct) is much larger than the energy splitting in tetrahedral complexes (Δ_tet). The tetrahedral complexes usually have 4/9 Δ_oct.

The color of a complex is determined by the color of light it absorbs. The complementary color of the absorbed light is the color we see. The energy and wavelength of absorbed light are inversely related, which means that a higher energy transition requires a shorter wavelength. As we know, the energy of light increases in the order: red < orange < yellow < green < blue < violet. Based on the spectrochemical series, the complex with NH_3 ligands should experience the largest energy splitting, followed by the one with H_2O ligands. The complex with Cl^− ligands will have the lowest energy splitting as it is a tetrahedral complex. 1. \([Co(NH_3)_6]^{2+}\): The largest energy splitting - absorbs lower wavelength (blue/violet) - appears yellow/pale orange (complementary color). 2. \([Co(H_2O)_6]^{2+}\): The intermediate energy splitting - absorbs mid-wavelength (green/blue) - appears pink/red (complementary color). 3. \([CoCl_4]^{2-}\): The smallest energy splitting - absorbs higher wavelength (red/orange) - appears blue/green (complementary color). So, the colors of the complexes are: - \([Co(NH_3)_6]^{2+}\): Yellow - \([Co(H_2O)_6]^{2+}\): Pink - \([CoCl_4]^{2-}\): Blue