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Chapter 21: Problem 45

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 \(-\mathrm{Cr}(\mathrm{acac})_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\), and \(\mathrm{Cr}(\mathrm{acac})_{3} ?\)

Short Answer

Expert verified
Only the cis-Cr(acac)₂(H₂O)₂ complex is optically active, as it possesses a chiral center with non-superposable mirror images. The other two complexes, trans-Cr(acac)₂(H₂O)₂ and Cr(acac)₃, are not optically active because they are achiral.

Step by step solution

01

Examine the structure of cis-Cr(acac)₂(H₂O)₂

For the cis-Cr(acac)₂(H₂O)₂ complex, we have a Cr(III) metal center, coordinating two bidentate acac ligands and two H₂O ligands in a cis configuration. To visualize the structure, we can represent the acac ligands as A, and the H₂O ligands as B. For simplicity, we display coordination as cis-ABCrAB (cis configuration of A and B ligands). This structure possesses a non-superposable mirror image, making it chiral and optically active.
02

Examine the structure of trans-Cr(acac)₂(H₂O)₂

For the trans-Cr(acac)₂(H₂O)₂ complex, we still have a Cr(III) metal center, coordinating two bidentate acac ligands and two H₂O ligands, but this time in a trans configuration. This structure can be represented as trans-ABC rAB (trans configuration of A and B ligands). This structure possesses a mirror plane, thus it is superposable onto its mirror image, making it achiral and not optically active.
03

Examine the structure of Cr(acac)₃

The Cr(acac)₃ complex has a Cr(III) metal center, coordinating three bidentate acac ligands. This complex adopts an octahedral geometry, which can be represented as AAACrAAA. This complex has no non-superposable mirror images, making it achiral and not optically active.
04

Conclude which complexes are optically active

From our analysis, only the cis-Cr(acac)₂(H₂O)₂ complex is optically active because it possesses a chiral center with non-superposable mirror images. The other two complexes, trans-Cr(acac)₂(H₂O)₂ and Cr(acac)₃ do not show optical activity as they are achiral.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Bidentate Ligand
In coordination chemistry, a bidentate ligand is a molecule that can form two bonds to a metal ion simultaneously. The term 'bi' indicates two, and 'dentate' refers to teeth, metaphorically suggesting that the ligand 'bites' the metal at two points. An example of a bidentate ligand is acetylacetone (acacH), which loses a proton to become acac- and coordinates to a metal through both its oxygen atoms.

This dual bonding creates a five-membered ring together with the metal ion, which can add to the stability of the complex through what's known as the chelate effect. The presence of bidentate ligands can greatly influence the overall geometry and sometimes the optical activity of a transition metal complex, as seen in compounds like cis-Cr(acac)2(H2O)2.
Chirality
Chirality is a geometric property where an object is not superposable on its mirror image, akin to how left and right hands are mirror images but not identical. In chemistry, a molecule is considered chiral if it can exist in two forms (enantiomers), which are non-superposable mirror images of each other.

These enantiomers have identical physical properties except for the way they rotate plane-polarized light; one isomer will rotate it to the right (dextrorotatory), and the other to the left (levorotatory). In the context of transition metal complexes, chirality arises due to the spatial arrangement of ligands around the metal center, as in the cis-Cr(acac)2(H2O)2 complex, which possesses a non-superposable mirror image and is thus optically active.
Transition Metal Coordination
Transition metal coordination refers to the process where transition metal ions form complexes with ligands. These ligands donate electron pairs to fill the d-orbitals of the metal ion, resulting in a transition metal complex. Transition metals have a unique capacity for forming diverse coordination compounds due to their varied oxidation states and the spatial arrangement of their d-orbitals.

The arrangement of ligands around the metal center in a specific geometric pattern (such as octahedral, tetrahedral, or square planar) is crucial for determining the properties of the complex, including its reactivity, color, and magnetic and optical characteristics. For instance, the cis and trans isomers of Cr(acac)2(H2O)2 differ dramatically in optical activity due to the different spatial arrangements of the acac ligands and water molecules.

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Most popular questions from this chapter

You isolate a compound with the formula \(\mathrm{PtCl}_{4} \cdot 2 \mathrm{KCl}\). From electrical conductance tests of an aqueous solution of the compound, you find that three ions per formula unit are present, and you also notice that addition of \(\mathrm{AgNO}_{3}\) does not cause a precipitate. Give the formula for this compound that shows the complex ion present. Explain your findings. Name this compound.

The carbonate ion \(\left(\mathrm{CO}_{3}^{2-}\right)\) can act as either a monodentate or a bidentate ligand. Draw a picture of \(\mathrm{CO}_{3}^{2-}\) coordinating to a metal ion as a monodentate and as a bidentate ligand. The carbonate ion can also act as a bridge between two metal ions. Draw a picture of a \(\mathrm{CO}_{3}^{2-}\) ion bridging between two metal ions.

Ethylenediaminetetraacetate \(\left(\mathrm{EDTA}^{4-}\right)\) is used as a complexing agent in chemical analysis with the structure shown in Figure 21.7. Solutions of EDTA \(^{4-}\) are used to treat heavy metal poisoning by removing the heavy metal in the form of a soluble complex ion. The complex ion essentially eliminates the heavy metal ions from reacting with biochemical systems. The reaction of EDTA \(^{4-}\) with \(\mathrm{Pb}^{2+}\) is \(\mathrm{Pb}^{2+}(a q)+\mathrm{EDTA}^{4-}(a q) \rightleftharpoons \mathrm{PbEDTA}^{2-}(a q) \quad K=1.1 \times 10^{18}\) Consider a solution with \(0.010 \mathrm{~mol} \mathrm{~Pb}\left(\mathrm{NO}_{3}\right)_{2}\) added to \(1.0 \mathrm{~L}\) of an aqueous solution buffered at \(\mathrm{pH}=13.00\) and containing \(0.050\) \(M \mathrm{Na}_{4} \mathrm{EDTA} .\) Does \(\mathrm{Pb}(\mathrm{OH})_{2}\) precipitate from this solution? \(\left(K_{\text {?? }}\right.\) for \(\left.\mathrm{Pb}(\mathrm{OH})_{2}=1.2 \times 10^{-15} .\right)\)

What is the electron configuration for the transition metal ion(s) in each of the following compounds? a. \(\left(\mathrm{NH}_{4}\right)_{2}\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2} \mathrm{Cl}_{4}\right]\) b. \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{2}\left(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\right)_{2}\right] \mathrm{I}_{2}\) c. \(\mathrm{Na}_{2}\left[\mathrm{TaF}_{7}\right]\) d. \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{I}_{2}\right]\left[\mathrm{Pt} \mathrm{I}_{4}\right]\) Pt forms \(+2\) and \(+4\) oxidation states in compounds.

The compound cisplatin, \(\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\), has been studied extensively as an antitumor agent. The reaction for the synthesis of cisplatin is: $$\mathrm{K}_{2} \mathrm{PtCl}_{4}(a q)+2 \mathrm{NH}_{3}(a q) \longrightarrow \mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}(s)+2 \mathrm{KCl}(a q)$$ Write the electron configuration for platinum ion in cisplatin. Most \(d^{8}\) transition metal ions exhibit square planar geometry. With this and the name in mind, draw the structure of cisplatin.
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