The compound $\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6} \mathrm{Cl}_{2}$ is green, whereas \(\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6} \mathrm{Cl}_{2}\) is violet. Predict the predominant color of light absorbed by each compound. Which compound absorbs light with the shorter wavelength? Predict in which compound \(\Delta\) is greater and whether \(\mathrm{H}_{2} \mathrm{O}\) or \(\mathrm{NH}_{3}\) is the stronger field ligand. Do your conclusions agree with the spectrochemical series?

When a compound appears green, it's because it reflects or transmits green light, which indicates that the compound absorbs the color complementary to green. Similarly, if a compound appears violet, it reflects or transmits violet light and absorbs the color that is complementary to violet.

For green, the complementary color is red, which is on the longer wavelength end of the spectrum. For violet, the complementary color is yellow, which is on the shorter wavelength end of the spectrum.

Since the green compound primarily absorbs red light and the violet compound primarily absorbs yellow light, we can conclude that the violet compound (\(\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6} \mathrm{Cl}_{2}\)) absorbs light with a shorter wavelength than the green compound (\(\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6} \mathrm{Cl}_{2}\)).

Since the violet compound (\(\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6} \mathrm{Cl}_{2}\)) absorbs shorter wavelengths, it must have a greater crystal field splitting energy, \(\Delta\), because shorter wavelengths of light have higher energy. Therefore, the stronger field ligand must be \(\mathrm{NH}_{3}\), as it results in a greater \(\Delta\) in the complex.

In the spectrochemical series, ligands are ordered based on their ability to produce large crystal field splitting energies. When comparing the ligands given in the problem, we would find that \(\mathrm{NH}_{3}\) indeed is a stronger field ligand than \(\mathrm{H}_{2} \mathrm{O}\), confirming our conclusion from step 4. To summarize: 1. The green compound \(\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6} \mathrm{Cl}_{2}\) predominantly absorbs red light, whereas the violet compound \(\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6} \mathrm{Cl}_{2}\) predominantly absorbs yellow light. 2. The violet compound absorbs light with a shorter wavelength. 3. The stronger field ligand is \(\mathrm{NH}_{3}\), and the complex with \(\mathrm{NH}_{3}\) has a greater crystal field splitting energy, \(\Delta\). 4. Our conclusions agree with the spectrochemical series.