Problem 25 - General Chemistry

Question

What is the spectrochemical series? Use the ligands \(\mathrm{CN}^{-}, \mathrm{H}_{2} \mathrm{O}, \mathrm{Cl}^{-}\), and \(\mathrm{NH}_{3}\) to illustrate the term. Then arrange them in order, describing the meaning of this order.

Step-by-Step Solution

Answer

The spectrochemical series ranks ligands by field strength: \(\mathrm{Cl}^{-} \lt \mathrm{H}_2\mathrm{O} \lt \mathrm{NH}_3 \lt \mathrm{CN}^{-}\).

Step 1: Define Spectrochemical Series

The spectrochemical series is an empirically derived ranking of ligands based on the strength of the field they produce when they coordinate to a central metal ion in a complex. This series orders ligands from weak field to strong field based on their ability to split the d-orbitals of the central metal ion.

Step 2: Identify Given Ligands

The ligands given in the problem are \(\mathrm{CN}^{-}, \mathrm{H}_{2} \mathrm{O}, \mathrm{Cl}^{-}\), and \(\mathrm{NH}_{3}\). These ligands can be arranged in the spectrochemical series order to understand their field strength.

Step 3: Arrange Ligands in Spectrochemical Order

To arrange the ligands, we consult the spectrochemical series: \(\mathrm{Cl}^{-} \lt \mathrm{H}_2\mathrm{O} \lt \mathrm{NH}_3 \lt \mathrm{CN}^{-}\).This order starts with the weakest field ligand \(\mathrm{Cl}^{-}\) and ends with the strongest field ligand \(\mathrm{CN}^{-}\).

Step 4: Explain the Order Meaning

The order indicates the ability of each ligand to cause splitting of the d-orbitals of the central metal ion. Weak field ligands like \(\mathrm{Cl}^{-}\) cause small d-orbital splitting, leading to high-spin complexes. Strong field ligands like \(\mathrm{CN}^{-}\) cause large d-orbital splitting, resulting in low-spin complexes.

Key Concepts

Ligand Field Strength

When ligands bind to a central metal ion, they create an electric field that influences the metal's properties. This is known as the ligand field. The strength of the ligand field depends on the ligands themselves and affects the degree of splitting of the metal's d orbitals. In the spectrochemical series, ligands are ranked from weak to strong based on how they affect this splitting. This ranking is not random but reflects their ability to donate or share electrons with the metal ion. For example, ligands like chloride (\(\mathrm{Cl}^{-}\)) have weaker fields, while cyanide (\(\mathrm{CN}^{-}\)) has a stronger field. Water (\(\mathrm{H}_2\mathrm{O}\)) and ammonia (\(\mathrm{NH}_3\)) fall somewhere in between. The strength of the field impacts how electrons are arranged within the metal's d orbitals and determines whether the complex is high-spin or low-spin.

d-Orbital Splitting

In chemistry, the d orbitals of transition metal ions can exhibit splitting when ligands bind to the metal. The ligands' electric fields cause the degeneration of these energy levels into two different groups. The degree of splitting depends on the nature of the ligands. In the spectrochemical series, as you move from weak field ligands to strong field ligands, the splitting increases. This is a critical concept because the distribution of electrons between the splitted levels influences the magnetic properties and color of a complex. For instance, strong field ligands like \(\mathrm{CN}^{-}\) can result in significant splitting, altering the energy gap between the groups. This may lead to changes in how the electron pairs are formed within the orbitals, which in turn influences whether the complex is a high-spin or low-spin configuration.

High-spin and Low-spin Complexes

When considering high-spin and low-spin complexes, you're examining how electrons fill the split d orbitals in a metal ion. This depends on both the splitting produced by the ligand field and the electron pairing energy. Weak field ligands, like \(\mathrm{Cl}^{-}\), cause only slight splitting of the d orbitals. As a result, electrons prefer not to pair up and occupy higher energy orbitals, creating a high-spin complex. These complexes are characterized by unpaired electrons, which often make them magnetic. Conversely, strong field ligands such as \(\mathrm{CN}^{-}\), cause a large energy gap or splitting between orbitals. In such cases, electrons will pair up in the lower energy orbitals rather than transfer to a higher energy level, leading to a low-spin complex. These complexes often show lower magnetism due to the paired electrons. Understanding the difference between high-spin and low-spin is essential for predicting the properties of metal complexes, including their color, magnetism, and reactivity.