Question

The \(\left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) ion has an absorption maximum at about \(725 \mathrm{~nm}\), whereas the \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}\) ion absorbs at about \(570 \mathrm{~nm} .\) Predict the color of each ion. (b) The \(\left[\mathrm{Ni}(\mathrm{en})_{3}\right]^{2+}\) ion absorption maximum occurs at about \(545 \mathrm{~nm}\), and that of the [Ni(bipy) \(\left._{3}\right]^{2+}\) ion occurs at about \(520 \mathrm{~nm}\). From these data, indicate the relative strengths of the ligand fields created by the four ligands involved.

Short answer

The predicted colors of the ions are: \(\left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\): Green and \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}\): Violet. The order of ligand field strength for the involved ligands is: bipy > en > \(\mathrm{NH}_{3}\) > \(\mathrm{H}_{2} \mathrm{O}\).

Step-by-step solution

Determine the absorbed colors from given wavelengths

To predict the color of the complexes, we must first find which color each complex absorbs from the visible spectrum. Visible light ranges from approximately 400nm (violet) to 700nm (red). The absorbed color corresponds to the energy of absorbed photons, and the complex will appear to be the complementary color on the color wheel. Recall that for the given complexes, the absorption maxima are: - \(\left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) at \(725 \mathrm{~nm}\) - \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}\) at \(570 \mathrm{~nm}\) - \(\left[\mathrm{Ni}(\mathrm{en})_{3}\right]^{2+}\) at \(545 \mathrm{~nm}\) - \(\left[\mathrm{Ni}(\mathrm{bipy})_{3}\right]^{2+}\) at \(520 \mathrm{~nm}\)

Determine the complementary colors

Now we need to find the complementary colors of the absorbed colors using a color wheel. The complementary color will be the color directly opposite to the absorbed color on the wheel. For the given complexes, we have: - \(\left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\): Absorbs red light (725nm) -> Complementary color: Green - \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}\): Absorbs yellow-green light (570nm) -> Complementary color: Violet So, the predicted colors of these ions are: - \(\left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\): Green - \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}\): Violet

Determine the relative strengths of the ligand fields

We can compare the absorption maxima of the given complexes to determine the relative strengths of the ligand fields. A higher absorption maximum corresponds to a stronger ligand field, as it requires more energy to be absorbed. Order of ligand field strength (strongest to weakest) based on the given absorption maxima: 1. \(\left[\mathrm{Ni}(\mathrm{bipy})_{3}\right]^{2+}\) at \(520 \mathrm{~nm}\) 2. \(\left[\mathrm{Ni}(\mathrm{en})_{3}\right]^{2+}\) at \(545 \mathrm{~nm}\) 3. \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}\) at \(570 \mathrm{~nm}\) 4. \(\left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) at \(725 \mathrm{~nm}\) So, the order of ligand field strength for the involved ligands is: bipy > en > \(\mathrm{NH}_{3}\) > \(\mathrm{H}_{2} \mathrm{O}\).

Key concepts

Color Absorption

When light interacts with a chemical compound, certain wavelengths are absorbed, leading to the color we perceive. In complex ions, this absorption is primarily due to the interaction between light and the electronic structure of the metal and its ligands.
The specific wavelengths absorbed by a complex ion are key to understanding its color because an ion absorbs certain parts of the visible light spectrum.
The visible spectrum ranges from about 400 nm to 700 nm, covering colors from violet to red. Each complex has a characteristic absorption maximum, determining which color it absorbs from the visible spectrum.
For instance, the complex ion \([\mathrm{Ni}(\mathrm{H}_{2}\mathrm{O})_{6}]^{2+}\) absorbs light at about 725 nm, corresponding to the red part of the spectrum. As a result, the ion appears green, which is the complementary color of red. This principle applies to other complex ions as well, e.g., \([\mathrm{Ni}(\mathrm{NH}_{3})_{6}]^{2+}\) absorbs yellow-green light, making it appear violet.
Understanding this absorption process helps in predicting the observed color of complex ions based on their absorption spectrum.

Visible Spectrum

The visible spectrum consists of all the light that human eyes can perceive, with wavelengths from about 400 nm (violet) to 700 nm (red). Each color in this spectrum corresponds to a specific wavelength.
When a complex ion absorbs certain wavelengths, it will often appear as the complementary color on the color wheel. This wheel is a circular arrangement of colors, where opposite colors complement each other.
For example:

• Red (around 700-640 nm) has a complementary color of green.
• Yellow-green (around 570 nm) is complemented by violet.

In complex chemistry, using the color wheel and the knowledge of the visible spectrum allows us to predict the observed colors of complex ions by knowing the specific wavelengths they absorb.
This makes the visible spectrum a powerful tool in determining how different complexes will appear to the naked eye.

Ligand Strength

Ligand field strength indicates how much a ligand can influence the electronic environment of a metal center in a complex.
Stronger ligands create a greater energy difference between the electronic states of the metal ions, which impacts the absorption of light.
The relative strength of ligands in a complex can be compared using their absorption maxima. A lower absorption wavelength corresponds to a higher ligand field strength because more energy is needed for electronic transitions.
In our example:

• \([\mathrm{Ni}(\mathrm{bipy})_{3}]^{2+}\) has the strongest field at 520 nm.
• \([\mathrm{Ni}(\mathrm{en})_{3}]^{2+}\) follows closely with 545 nm.
• \([\mathrm{Ni}(\mathrm{NH}_{3})_{6}]^{2+}\) is next at 570 nm.
• \([\mathrm{Ni}(\mathrm{H}_{2}\mathrm{O})_{6}]^{2+}\) has the weakest field at 725 nm.

Understanding ligand strength helps predict the behavior of complexes under different conditions, such as their color and reactivity.

Complex Ions

Complex ions are formed when a central metal ion is surrounded by molecules or ions known as ligands. These ligands are bound to the central metal through coordinate covalent bonds.
The structure and properties of these complex ions are determined by several factors including the type of metal, the nature of the ligands, and the spatial arrangement of the ligands around the metal.
Each complex ion can exhibit unique properties, such as particular colors or reactivity due to the distribution of electrons between the metal and the ligands.
Utilizing Ligand Field Theory, we can understand how these electronic interactions occur, which in turn helps predict the absorption characteristics and colors of the complex ions. For instance, the nature of the ligand can cause changes in the electronic states of the metal center, leading to differences in the energy of absorbed light."
Complex ions show the diversity of coordination chemistry and how variations in ligands and structure lead to a wide range of observed properties.