Question

Acetylacetone, abbreviated acacH, is a bidentate ligand. It loses a proton and coordinates as acac \(^{-},\) as shown below, where \(\mathrm{M}\) is a transition metal: Which of the following complexes are optically active: cis-\(\mathrm{Cr}(\mathrm{acac})_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2},\) trans-Cr(acac) \(_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2},\) and \(\mathrm{Cr}(\mathrm{acac})_{3} ?\)

Short answer

The cis-Cr(acac)₂(H₂O)₂ and Cr(acac)₃ complexes are optically active, while the trans-Cr(acac)₂(H₂O)₂ complex is not optically active.

Step-by-step solution

Analyze cis-Cr(acac)₂(H₂O)₂

For the cis-Cr(acac)₂(H₂O)₂ complex, the ligands are arranged in an octahedral geometry (since there are a total of 6 ligands), with two acac ligands in a cis arrangement and two water molecules in a cis arrangement as well. Since the ligands are not arranged in a mirror symmetric structure and it possess a set of enantiomers, this complex is optically active.

Analyze trans-Cr(acac)₂(H₂O)₂

For the trans-Cr(acac)₂(H₂O)₂ complex, the ligands are still in an octahedral geometry. However, in this case, the two acac ligands are in a trans arrangement and two water molecules are in a trans arrangement. This configuration leads to a mirror symmetric structure and the complex does not have enantiomers. Thus, the trans-Cr(acac)₂(H₂O)₂ complex is not optically active.

Analyze Cr(acac)₃

For the Cr(acac)₃ complex, there are three acac ligands in an octahedral geometry. In this case, the complex configuration results in a set of enantiomers as it has a non-superimposable mirror image. Therefore, the Cr(acac)₃ complex is optically active. To sum up the results: cis-Cr(acac)₂(H₂O)₂: optically active trans-Cr(acac)₂(H₂O)₂: not optically active Cr(acac)₃: optically active

Key concepts

Bidentate Ligands

Bidentate ligands, like acetylacetone (acac), are fascinating molecules that can coordinate with metals through two donor atoms. Consider the term 'bidentate' – 'bi' meaning two and 'dentate' implying teeth. Essentially, they "bite" onto a metal ion at two sites. This dual binding provides additional stability to the complex compared to monodentate ligands that use only one donor atom.

Acetylacetone, specifically, binds via its oxygen atoms after losing a proton to form acac-. This ability to form two bonds at once leads to stronger bonding and alters the properties of the metal complex. Some key points about bidentate ligands include:

• Increased stability of the complex due to chelation, known as the chelate effect.
• Ability to form stable 5- or 6-membered rings with the metal center.
• Influence on the optical activity of the complex based on ligand orientation.

Understanding these characteristics help predict how a ligand like acac will interact and contribute to the optical properties of coordination complexes.

Octahedral Geometry

In coordination chemistry, octahedral geometry is one of the most common and important structures. It involves six ligands symmetrically arranged around a central metal atom, forming an octahedral shape. Imagine a globe surrounded by six symmetrically spaced cities – that's the general structure here.

Octahedral complexes can display distinct spatial arrangements, such as cis and trans configurations. For instance, when a ligand like acetylacetone (acac) and water are coordinated around chromium (Cr), they can adopt either a cis or trans configuration. Each configuration has different properties:

• Cis Configuration: Ligands are adjacent to each other, often resulting in chiral complexes that may have optical activity due to their non-superimposable mirror images.
• Trans Configuration: Opposite placements typically leading to non-chiral arrangements that are typically optically inactive.

The arrangement significantly influences the chemical and physical characteristics of the complex, including its optical activity.

Enantiomers

Enantiomers are like two hands – they are mirror images but not identical. In chemistry, this concept translates to molecules: two compounds are enantiomers if they are non-superimposable mirror images of each other. This property is crucial in determining optical activity.

In coordination complexes, the spatial arrangement of ligands can lead to enantiomeric forms. If a complex is unable to align exactly with its mirror counterpart, it is chiral and can exist as a pair of enantiomers. These pairs react differently to polarized light: one will rotate the light in a clockwise direction (dextrorotatory), and the other in a counterclockwise direction (levorotatory).

• Presence of enantiomers in a complex indicates optical activity.
• Cis configurations often lead to chiral, optically active complexes.
• Enantiomers are key in fields requiring precise molecular interactions, such as pharmaceuticals.

Understanding enantiomers helps in predicting optical behavior in coordination complexes.

Transition Metals

Transition metals are versatile elements found in the d-block of the periodic table and are known for their ability to form various coordination complexes. They have partially filled d orbitals which make them ideal for forming complex structures with ligands.

These metals can create complexes with unique properties, such as optical activity, depending on how ligands are arranged around them in structures like octahedral geometries. Chromium (Cr) is an example of a transition metal frequently involved in coordination chemistry.

• Transition metals can exhibit multiple oxidation states, allowing for diverse complex formation.
• They form coordination complexes with predictable geometries, including octahedral, tetrahedral, and square planar.
• Their complexes can display unique colors, magnetism, and optical activity based on their electronic structure and ligand arrangement.

Transition metals like chromium play a crucial role in forming structures capable of exhibiting optical activity, highlighting their significance in coordination chemistry.