Question

The complex ion \(\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{2+}\) has an absorption maximum at around \(800 \mathrm{nm} .\) When four ammonias replace water, \(\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}^{2+},\) the absorption maximum shifts to around \(600 \mathrm{nm} .\) What do these results signify in terms of the relative field splittings of \(\mathrm{NH}_{3}\) and \(\mathrm{H}_{2} \mathrm{O}\) ? Explain.

Short answer

The results signify that the field splitting caused by \(\mathrm{NH}_{3}\) is larger than that caused by \(\mathrm{H}_{2} \mathrm{O}\). This is indicated by the shorter absorption maximum wavelength of \(\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}^{2+}\) compared to \(\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{2+}\).

Step-by-step solution

Understanding the concept of field splitting in complex ions

In coordination chemistry and ligand field theory, field splitting refers to the splitting of degenerate energy levels of a metal ion in the presence of ligands. The energy gap between the higher and lower energy levels correlates to the wavelength of light that can be absorbed by the complex ion. Larger field splitting means larger energy difference, resulting in the absorption of shorter wavelengths (higher energy).

Analyzing the absorption maximum wavelengths of the two complex ions

We are given the absorption maximum wavelengths for the two complex ions: - \(\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{2+}\) absorbs light at around \(800 \mathrm{nm}\) - \(\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}^{2+}\) absorbs light at around \(600 \mathrm{nm}\) Since the absorption maximum wavelength of \(\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}^{2+}\) (600 nm) is shorter than that of \(\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{2+}\) (800 nm), we can infer that the field splitting for the complex ion with ammonia ligands is larger.

Comparing the field splittings of the two complex ions

We have established that the field splitting for \(\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}^{2+}\) is larger than that for \(\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{2+}\). From this, we can conclude that ammonia ligands (\(\mathrm{NH}_{3}\)) cause a larger field splitting than water ligands (\(\mathrm{H}_{2} \mathrm{O}\)). In summary, the results signify that the field splitting caused by \(\mathrm{NH}_{3}\) is larger than that caused by \(\mathrm{H}_{2} \mathrm{O}\). This is indicated by the shorter absorption maximum wavelength of \(\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}^{2+}\) compared to \(\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{2+}\).

Key concepts

Complex Ion

Complex ions are fascinating structures found within coordination chemistry. They are composed of a central metal ion bonded to surrounding molecules or ions known as ligands. These ligands can be neutral molecules like water ( H$_2$ O) or charged species. The central metal ion and its shells of ligands form a complex that can carry a charge, which can be positive, negative, or neutral depending on the makeup of the ligands.
A complex ion such as Cu(H$_2$O)$_6$$^{2+}$ consists of a copper ion in its center, surrounded by six water molecules. The existing bonds between the metal ion and its ligands are not purely ionic or covalent but represent a unique type of bonding called coordinate covalent bonding. Here, the ligands provide electron pairs to the metal, stabilizing the overall structure.
These complex ions are essential in various chemical processes and have unique properties, such as the ability to absorb specific wavelengths of light. This property is influenced by how the ligands interact with the metal ion, ultimately affecting the behavior and appearance of the complex ion in solutions. Understanding complex ions helps explain many reactions and characteristics of different metal-ligand systems.

Absorption Maximum

The absorption maximum of a complex ion refers to the specific wavelength of light that is most strongly absorbed by the complex. This phenomenon is critical because it provides insights into the energy levels of the metal ion influenced by the surrounding ligands.
For instance, in our exercise, we observe that the complex ion Cu(H$_2$O)$_6$$^{2+}$ absorbs light around 800 nm, while Cu(NH$_3$)$_4$(H$_2$O)$_2$$^{2+}$ shows a shift to 600 nm. This change indicates a smaller absorption maximum wavelength, signifying a larger energy difference between orbitals.
The shift to a shorter wavelength (higher energy) suggests that when ammonia replaces water as a ligand, the field splitting increases, leading to a stronger interaction and thus a higher energy absorption. This relationship is a fundamental concept in ligand field theory and is crucial for predicting the color and reactivity of complex ions. Analyzing absorption maxima helps chemists determine the nature and strength of ligand-metal interactions.

Coordination Chemistry

Coordination chemistry primarily concerns the study of complex ions and the interactions between metal centers and ligands. It explores how these interactions bring about various properties such as color changes, magnetism, and reactivity.
A central focus is on understanding field splitting—a phenomenon where degeneracy in metal ion orbitals is lifted due to the presence of ligands. The type of ligand and the geometry of the complex influence how much these orbitals are split. Field splitting dictates the energy gap between d-orbitals in the metal ion, thereby affecting the absorption of light and consequently the color of the complex.
In our context, substituting water with ammonia as a ligand in copper complexes changes the absorption maximum wavelengths due to field splitting. The ammonia, being a stronger ligand, causes a greater splitting, and thus influences the absorption from 800 nm to 600 nm. This shift is key for chemists to tailor the properties of complexes for specific applications, expanding the versatility of transition metal compounds in industrial and research settings.