Problem 88 - Chemistry: The Central Science in SI Units, Global Edition

Question

When Alfred Werner was developing the field of coordination chemistry, it was argued by some that the optical activity he observed in the chiral complexes he had prepared was due to the presence of carbon atoms in the molecule. To disprove this argument, Werner synthesized a chiral complex of cobalt that had no carbon atoms in it, and he was able to resolve it into its enantiomers. Design a cobalt(III) complex that would be chiral if it could be synthesized and that contains no carbon atoms. (It may not be possible to synthesize the complex you design, but we will not worry about that for now.)

Step-by-Step Solution

Answer

A possible chiral cobalt(III) complex that contains no carbon atoms is [Co(NH3)3Cl3]. This complex exhibits chirality due to different spatial arrangements of NH3 and Cl- ligands around the central cobalt ion. The complex has an octahedral geometry with a coordination number of 6, and its enantiomers are non-superimposable mirror images, emphasizing the presence of chirality.

Step 1: Understanding chirality

Chirality is a property of molecules where a molecule and its mirror image are non-superimposable, similar to left and right hands. Chiral molecules have one or more centers of chirality, usually a stereogenic carbon atom is involved with four different substituents (i.e., groups attached to the atom). However, metal complexes can also exhibit chirality, even though they may not contain stereogenic carbon atoms. A chiral metal complex typically involves a coordination number of four or higher.

Step 2: Understanding cobalt(III) complex

A cobalt(III) complex refers to a metal complex containing a central cobalt ion in the oxidation state of +3, Co(III), bonded to other atoms or groups, which are known as ligands. The coordination number is the number of bonds between the central ion and its ligands.

Step 3: Designing a chiral cobalt(III) complex

Since the desired complex must have no carbon atoms, we will focus on creating a chiral cobalt(III) complex with halide ligands or other inorganic ligands. Remember, the coordination number must be four or higher for the complex to exhibit chirality. In this context, the octahedral geometry (coordination number 6) seems to be the most suitable geometry. A possible chiral cobalt(III) complex to be synthesized would be [Co(NH3)3Cl3]. In this complex, the central cobalt(III) ion is bonded to three ammonia ligands (NH3) and three chloride ligands (Cl-). Overall, the complex is a neutral octahedral species and is chiral due to the different arrangement of NH3 and Cl- ligands around the cobalt center. Please note that the complex is hypothetical and may not be synthesized in reality.

Step 4: Visualization of the chiral cobalt(III) complex

It could be helpful to draw the structure of the chiral [Co(NH3)3Cl3] complex, emphasizing the octahedral arrangement of the ligands. To visualize the chirality, you can draw both enantiomers, non-superimposable mirror images. In conclusion, a possible chiral cobalt(III) complex that contains no carbon atoms is [Co(NH3)3Cl3]. This complex exhibits chirality due to different spatial arrangements of NH3 and Cl- ligands around the central cobalt ion.

Key Concepts

Cobalt(III) Complex

A cobalt(III) complex is a type of coordination compound where cobalt is the central metal ion. In these complexes, cobalt is in a +3 oxidation state, written as Co(III).
This means cobalt has lost three electrons, giving it a positive charge of three units. Cobalt is bonded to other atoms or groups, known as ligands, which can be ions or neutral molecules that donate electron pairs.

• The bonds between the cobalt ion and its ligands form what is known as the coordination sphere.
• These bonds are generally coordinate covalent bonds, where both electrons come from the ligands.

The coordination number, which is the number of ligand bonds to the central ion, can vary, but in many cobalt complexes like the one in our problem, it's six, leading to an octahedral geometry. This type of geometric structure allows for the possibility of chirality in some complexes, depending on how the ligands are arranged.

Optical Isomerism

Optical isomerism occurs when two molecules are nonsuperimposable mirror images of one another, much like your left and right hands. These isomers are known as enantiomers. Optical isomerism is a subtype of stereoisomerism and is crucial in many fields, including chemistry and pharmacology, due to the distinct properties of each isomer.

• Enantiomers have identical physical and chemical properties in a non-chiral environment, such as melting points and solubility, but they interact differently with polarized light.
• When placed in a polarized light, each enantiomer will rotate the plane of light in opposite directions - this is known as optical activity.

For a coordination complex to exhibit optical isomerism, it must be chiral, which means it must lack an internal plane of symmetry.
In coordination chemistry, chirality usually arises when the spatial arrangement of ligands around the central metal ion can create non-superimposable mirror images, like in our designed cobalt(III) complex.

Inorganic Ligands

In the context of coordination chemistry, ligands are ions or molecules that bind to a central metal ion to form a coordination complex. When we talk about inorganic ligands, we are referring to ligands that don't contain carbon, which sets them apart from organic ligands.

• Common inorganic ligands include halides like chloride (Cl^-), oxide (O^2-), and amines like ammonia (NH₃).
• They donate one or more pairs of electrons to the central metal ion, forming coordinate bonds.

Different types of ligands can greatly influence the chemistry of the central metal ion, affecting properties such as color, magnetic behavior, and, importantly, the geometry of the complex.
For instance, in the cobalt(III) complex discussed, ammonia and chloride are used as ligands to achieve chirality and meet the design requirement of having no carbon atoms present in the complex. As a result, [Co(NH₃)₃Cl₃] can be considered as a representative complex illustrating how inorganic ligands work in creating potentially chiral structures.