Question

A student in 1895 prepared three coordination compounds containing chromium, with the following properties: $$ \begin{array}{llc} & & \text { Cl }^{-} \text {Ions in Solution } \\ \text { Formula } & \text { Color } & \text { per Formula Unit } \\ \hline \text { (a) } \mathrm{Cr} \mathrm{Cl}_{3} \cdot 6 \mathrm{H}_{2} \mathrm{O} & \text { Violet } & 3 \\ \text { (b) } \mathrm{Cr} \mathrm{Cl}_{3} \cdot 6 \mathrm{H}_{2} \mathrm{O} & \text { Light green } & 2 \\ \text { (c) } \mathrm{Cr} \mathrm{Cl}_{3} \cdot 6 \mathrm{H}_{2} \mathrm{O} & \text { Dark green } & 1 \end{array} $$ Write modern formulas for these compounds, and suggest a method for confirming the number of \(\mathrm{Cl}^{-}\) ions present in solution in each case.

Short answer

Cr(H₂O)₆Cl₃, CrCl(H₂O)₅Cl₂, CrCl₂(H₂O)₄Cl; use AgNO₃ for chloride ion testing.

Step-by-step solution

Understanding the Results

The table presents three coordination compounds with chromium, each exhibiting a different color and number of chloride ions in solution. All three compounds have the formula \( \text{CrCl}_3 \cdot 6 \text{H}_2\text{O} \), but the number of chloride ions in solution varies: 3 for the violet complex, 2 for the light green, and 1 for the dark green.

Analyzing Compound (a)

For the violet compound, having 3 chloride ions in solution suggests that all three chloride ions are ionic and dissociate completely in water. This implies the coordination number of chromium accounts only for water molecules: \( [\text{Cr(H}_2\text{O})_6]^{3+} + 3\text{Cl}^- \). This occurs when all chlorides are outside the coordination sphere.

Analyzing Compound (b)

For the light green compound, with 2 chloride ions in solution, it means two chloride ions are ionic and dissociate in water, while one chloride is coordinated to the metal center. The compound is therefore: \( [\text{CrCl(H}_2\text{O})_5]^{2+} + 2\text{Cl}^- \). This places one chloride inside the coordination sphere.

Analyzing Compound (c)

For the dark green compound, having only 1 chloride ion in solution suggests only one is ionic. This means two chloride ions are coordinated to the central chromium, resulting in: \( [\text{CrCl}_2(\text{H}_2\text{O})_4]^+ + \text{Cl}^- \). Two chlorides are now within the coordination sphere.

Modern Formulas

The modern molecular formulas for the compounds are as follows: \((a)\ [\text{Cr(H}_2\text{O})_6]\text{Cl}_3\), \((b)\ [\text{CrCl(H}_2\text{O})_5]\text{Cl}_2\), \((c)\ [\text{CrCl}_2(\text{H}_2\text{O})_4]\text{Cl}\).

Confirmation Method

To confirm the number of \(\text{Cl}^-\) ions, conduct a chloride ion precipitation test using silver nitrate (\(\text{AgNO}_3\)). The number of \(\text{AgCl}\) precipitate molecules formed reflects the number of free \(\text{Cl}^-\) ions in the solution.

Key concepts

Coordination Compounds

Coordination compounds are a fascinating area of chemistry where transition metals form complexes with ligands. Ligands are ions or molecules that can donate a pair of electrons to a metal. In-depth understanding of coordination compounds reveals how these structures are held together and exhibit unique properties, such as color changes.
A core feature of these compounds is the coordination number, which indicates the number of ligand atoms bonded to the central metal ion. This characteristic is often related to the metal's oxidation state and the properties of the ligands involved.
Coordination compounds play a crucial role in various applications, including catalysis, material science, and medicine. For example, they are used in chemotherapy drugs, pigments, and even as catalysts in industrial processes.

Chromium Complexes

Chromium is a well-known transition metal, making it a perfect candidate for forming coordination complexes. Chromium can have various oxidation states, but in coordination chemistry, it often forms complexes in its +3 oxidation state. This provides interesting opportunities for studying the different behaviors of chromium complexes, such as their colors in solutions.
This was observed in the 1895 experiment where three different chromium compounds each exhibited unique color properties: violet, light green, and dark green. These differences arise from the number of chloride ions dissociated in solution and the chromium's coordination environment.
The color change is a direct result of differing absorption of light, which depends on the ligand types and arrangement in the coordination sphere around chromium. Such experiments not only advance the understanding of chromium's electronic properties but also provide insights into the broader theory of transition metal complexes.

Chloride Ion Dissociation

Chloride ion dissociation in coordination compounds is important because it affects the resulting properties of the compound, including solubility and color. When chloride ions dissociate from a metal complex, they become free ions in solution, affecting the overall ionic strength and the electrical neutrality of the system.
In the provided exercise, the difference in the number of dissociated chloride ions among the three chromium complexes shows how they are distributed in the coordination sphere. This number reflects the degree of ionic versus covalent character of the chloride's bond with chromium.
For example, three free chloride ions suggest an entirely ionic interaction, while fewer free chloride ions indicate some are held covalently in the coordination sphere. A common method to test for these free ions is the silver nitrate precipitation test, which provides a visual indicator of the chloride ions available in the solution.

Coordination Sphere

The coordination sphere of a coordination compound is the portion of the structure directly bonded to the central metal atom. It consists of the central atom and its surrounding ligands, which can be both neutral molecules and charged ions. The coordination sphere defines the geometric arrangement of the ligands and is responsible for many of the complex's properties.
In the context of chromium complexes, we see that the coordination sphere influences the number of chloride ions able to dissociate in solution. The compound's color variations align with changes in the coordination sphere as ligands are substituted or repositioned.
This sphere and the spatial orientation of its constituents are crucial in determining the photophysical and chemical behaviors of coordination compounds. Understanding the coordination sphere is essential for predicting how these compounds will interact with their environments, whether in biological systems or industrial applications.