Problem 88 - Chemistry The Central Science

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 chiral cobalt(III) complex with no carbon atoms can be designed using ammonia (NH3) as a monodentate ligand and nitrite (NO2-) as a bidentate ligand. The complex would have an octahedral geometry and can be represented as: \[ \text{Co}(\text{NH}_3)_3(\text{NO}_2-\text{ONO})^{2+} \]

Step 1: 1. Understanding Chirality

A chiral molecule has a non-superimposable mirror image, which means it cannot be superimposed onto its mirror image. In coordination complexes, chirality often arises when the central metal is bonded to ligands in a non-planar arrangement, such as in an octahedral or tetrahedral geometry.

Step 2: 2. Determine the oxidation state of Cobalt

Since the given metal is cobalt(III), it has an oxidation state of +3.

Step 3: 3. Choose a coordination number and geometry

For a chiral complex, we should choose a non-planar geometry. An octahedral geometry is a common choice for cobalt(III) complexes as it allows for a coordination number of 6.

Step 4: 4. Selecting ligands that do not contain carbon atoms

We need to choose ligands that do not have carbon atoms and can form a chiral complex. One option is to use ammonia (NH3) as a monodentate ligand and nitrite (NO2-) as a bidentate ligand, which can bind to cobalt in two different ways: either as a nitro group (NO2) with nitrogen binding to the central metal or as a nitrito group (ONO) with oxygen binding to the central metal.

Step 5: 5. Design the chiral cobalt(III) complex

To create a chiral complex, we can use three ammonia ligands and one nitrite ligand, binding cobalt in a bidentate fashion. The resulting complex would have an octahedral geometry and be chiral: \[ \text{Co}(\text{NH}_3)_3(\text{NO}_2-\text{ONO})^{2+} \] This chiral cobalt(III) complex would have the required non-carbon-containing ligands and, while it may not be possible to synthesize it, it would fulfill the requirements of the exercise.