Question

Which of the complexes shown here are chiral? [Section 23.4] $$ \mathrm{Cr} \quad \mathrm{Cr}=\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2} \quad \Theta=\mathrm{Cl} \quad \theta=\mathrm{NH}_{3} $$ (1) ( (3) (4)

Short answer

None of the given complexes (Complex 1: Cr, Complex 2: Cr(NH2CH2CH2NH2), Complex 3: Cr(Theta), and Complex 4: Cr(theta)) are chiral as they do not exhibit required structural asymmetry or are not complete complexes.

Step-by-step solution

Complex 1: Cr (This is only the central metal atom)

Since there is only a central metal atom and no ligand, it is not a chiral complex.

Complex 2: Cr(NH2CH2CH2NH2)

In this case, the Cr is attached to a bidentate ligand (NH2CH2CH2NH2). It forms a five-membered chelate ring, and there is no other ligand to form a complex. Thus, this complex is a mono-dentate complex and is not chiral.

Complex 3: Cr(Theta) - Cr with ligand Theta (Cl)

Since a chlorine ligand is monodentate, the complex could form six-coordinate octahedral complexes. However, with only one ligand, it cannot form a complete complex and thus is not considered chiral.

Complex 4: Cr(theta) - Cr with ligand theta (NH3)

In this case, the Cr is attached to a monodentate ligand (NH3). Ammonia (NH3) has a lone pair of electrons that can act as a donor site. Thus, Cr(NH3) can form a complete complex in the octahedral geometry. However, six-coordinate octahedral complexes with identical ligands attached to the central metal ion are achiral. Since there is no other ligand to give this complex any asymmetricity, it is not chiral. In conclusion, none of the given complexes are chiral.

Key concepts

Coordination Chemistry

Coordination chemistry is the study of compounds that feature a central metal atom or ion bonded to one or more ligands. These ligands can be ions like halides, or molecules that can donate a pair of electrons to the metal to establish a coordinate covalent bond. The resulting structures are called coordination compounds or complexes.

The central metal ion or atom typically possesses an incomplete d-orbital that allows it to accept electrons from its ligands. This interaction gives rise to a diverse array of possible geometries and properties of the complexes, depending on factors such as the number and type of ligands bonded to the metal, and the oxidation state of the metal itself.

Complexes can be described by their coordination number, which refers to the number of ligand atoms that coordinate to the central metal. Common coordination geometries include linear, tetrahedral, square planar, and octahedral, each offering different spatial arrangements that play a crucial role in the overall behavior of the complex.

Chirality in Complexes

Chirality in chemistry refers to the property of a molecule that cannot be superimposed on its mirror image, much like how your right and left hands are mirror images but not identical. Chiral complexes, therefore, have no plane of symmetry and no center of inversion; they are non-superimposable on their mirror images.

In coordination chemistry, chirality typically arises when the spatial arrangement of the ligands around the central metal atom is such that it leads to an asymmetric, three-dimensional structure. These chiral complexes often exhibit interesting optical properties, such as the ability to rotate plane-polarized light, a phenomenon known as optical activity.

The chirality of a complex can have significant implications in various fields, including pharmaceutical chemistry, where the chirality of a drug molecule can affect its therapeutic effects and side effects. Hence, understanding and controlling chirality is essential for the design of specific drugs and materials.

Ligands and Chelation

Ligands are ions or molecules that can donate a pair of electrons to a central metal atom or ion in a coordination compound. They range from simple ions like chloride (Cl-) to complex organic molecules. The strength and nature of the metal-ligand bond are crucial in determining the stability and reactivity of the complex.

Chelation occurs when a ligand forms multiple bonds to a single metal center, effectively 'grabbing' the metal ion with two or more donor atoms. These ligands are known as chelating agents. Bidentate ligands, for example, have two donor atoms which can attach to the metal atom or ion, forming a five-membered or six-membered chelate ring. Chelate rings are commonly five or six atoms in size because such rings are particularly stable.

Chelation enhances the stability of the metal complex due to the chelate effect, which is a result of the entropy increase when a chelate ligand displaces several monodentate ligands and forms a ring structure. Additionally, the rigidity of the ring can lead to specific geometric configurations that may influence the properties of the complex, including its potential chirality.