Question

The reaction of the octahedral complex \(\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{NO}_{2}\right)_{3}\) with \(\mathrm{HCl}\) yields a complex \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{H}_{2} \mathrm{O}\right) \mathrm{Cl}_{2}\right]^{+}\) in which the two chloride ligands are trans to one another. (a) Draw the two possible stereoisomers of the starting material \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{NO}_{2}\right)_{3}\right] .\) (All three \(\mathrm{NO}_{2}^{-}\) ligands are bonded to Co through the \(\mathrm{N}\) atom. ) (b) Assuming that the \(\mathrm{NH}_{3}\) groups remain in place, which of the two starting isomers could give rise to the observed product?

Short answer

The 'mer' isomer forms the observed product.

Step-by-step solution

Identify the Octahedral Complex

The starting complex is the octahedral complex \([\text{Co}(\text{NH}_3)_3(\text{NO}_2)_3]\). Since it's octahedral, it has 6 coordination sites, with three \( \mathrm{NH}_3 \) and three \( \mathrm{NO}_2 \) ligands bonded through the nitrogen atom.

Understand the Geometry of Isomers

In the octahedral complex, the possible stereoisomers are 'facial' (fac) and 'meridional' (mer). In the fac isomer, the same types of ligands occupy one face of the octahedron, forming an equilateral triangle. In the mer isomer, the same ligands are arranged along a meridian plane, involving one axial and two equatorial positions.

Draw the "fac" Isomer

In the 'fac' isomer of the complex, all three \( \mathrm{NO}_2 \) ligands are adjacent, occupying the octahedron's corners \( (1, 2, 3) \). They form a triangle on one face of the octahedron, with the \( \mathrm{NH}_3 \) ligands filling the remaining positions.

Draw the "mer" Isomer

In the 'mer' isomer, the \( \mathrm{NO}_2 \) ligands are spread, such that two occupy opposite sides and the third is adjacent on the same side. This gives a linear arrangement along one axis and an L-shape across different planes inside the octahedron.

Determine Which Isomer Forms the Observed Product

The resulting complex has two chloride ions in the trans position, implying that one of the \( \mathrm{NO}_2 \) groups converting to water during the reaction must be in a position that transfers the other \( \mathrm{NO}_2 \) groups trans in the product. Thus, only the 'mer' isomer can arrange the Cl ligands trans as required in the final complex, maintaining the pattern of replacements.

Key concepts

Coordination Chemistry

Coordination chemistry is a branch of chemistry focused on the study of coordination complexes. These complexes are molecules that contain a central atom, usually a metal, surrounded by molecules or ions called ligands. In our exercise, the central metal atom is cobalt (Co). The ligands are ammonia (\( \text{NH}_3 \)) and nitrite (\( \text{NO}_2^-\)), which are bonded to cobalt through nitrogen atoms.

Coordination complexes are characterized by their coordination number, which indicates how many ligands are directly bonded to the metal center. For an octahedral complex such as \([\text{Co}(\text{NH}_3)_3(\text{NO}_2)_3]\), the metal center is surrounded by six ligands. This gives the complex its octahedral geometry.- Coordination Complex: A compound with a central atom and surrounding ligands.- Ligand: An ion or molecule bonded to the central metal atom.Understanding these basics of coordination chemistry is crucial for studying the behavior and properties of complexes like those involved in the given reaction.

Stereoisomers

In coordination chemistry, stereoisomers are molecules that have the same chemical formula and sequence of bonded atoms, but different spatial arrangements. For octahedral complexes, like \([\text{Co}(\text{NH}_3)_3(\text{NO}_2)_3]\), there are two primary stereoisomers: the "facial" (fac) isomer and the "meridional" (mer) isomer.

- **Facial (fac) Isomer:** In this isomer, three identical ligands occupy adjacent positions, effectively forming the corners of a triangle. In simpler terms, all three are on one face of the octahedron. This positioning gives the complex a specific three-faced symmetry.
- **Meridional (mer) Isomer:** This isomer has ligands arranged along one plane, spanning diagonally across the complex. Two of the identical ligands are opposite each other, with the third switching planes, giving this an L-shape arrangement.Understanding these arrangements helps in determining chemical reactivity and predicting the formation of certain products, as the exercise illustrates when determining the product stereo configuration.

Ligand Substitution

Ligand substitution is a fundamental concept in coordination chemistry where one or more ligands in a complex are replaced by another ligand. The process can significantly alter the properties and behavior of the complex.

In the exercise, ligand substitution occurs when a nitrite ligand in the original complex \([\text{Co}(\text{NH}_3)_3(\text{NO}_2)_3]\) is replaced by water (\(\mathrm{H}_2O\)) and chlorine ions (Cl\(^-\)) from HCl. This transformation leads to the formation of a new complex \( [\text{Co}(\text{NH}_3)_3(\mathrm{H}_2O)\text{Cl}_2]^{+} \).

- **Reaction Detail:** The key observation is the trans position of the chloride ions, meaning they are located directly opposite each other in the complex. This positional change is influenced by the starting geometric arrangement of the ligands.- **Stereochemistry Influence:** Based on the starting isomers, the availability of positions for substitution by new ligands can result in different products, hence why only the 'mer' isomer configuration correctly predicts the observed product outcome with trans positions for chlorides.

Octahedral Geometry

Octahedral geometry is a common structural form for coordination complexes with six ligands bound to a central metal atom. The name comes from the geometric shape, an octahedron, which has eight faces.

In the octahedral complex \([\text{Co}(\text{NH}_3)_3(\text{NO}_2)_3]\), the six ligands are positioned symmetrically around the cobalt atom. This arrangement is significant because it affects the complex's chemical properties, stability, and reactivity.- **Symmetrical Arrangement:** The geometry allows for different configurations of ligands, which leads to the possibility of creating stereoisomers, such as the 'fac' and 'mer' described previously.- **Complex Stability:** The spatial arrangement affects the stability of the complex, making certain reactions or substitutions more favorable, as seen with the trans substitution of chlorides in the product complex from our exercise.Understanding octahedral geometry helps in predicting the properties and behaviors of metal complexes, especially when embarking on more complex chemical reaction scenarios.