Problem 67 - 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 because of 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 won't worry about that for now.)

Step-by-Step Solution

Answer

A chiral cobalt(III) complex that contains no carbon atoms can be constructed using the following ligand set: chlorine (Cl-), ammonia (NH3), water (H2O), and a chiral anionic bidentate ligand derived from NO-OH, called nitrito-N,ONO (charge of -1). The complex can be represented as: \[ \text{[Co(Cl)(NH}_3\text{)(H}_2\text{O)(NO-OH)]^2- \] This complex exhibits chirality due to the tetrahedral arrangement of the four different ligands surrounding the cobalt(III) center and does not contain any carbon atoms.

Step 1: (Step 1: Determine the coordination number of cobalt(III))

Cobalt(III) typically forms octahedral complexes, which means it has a coordination number of 6. This means that cobalt(III) is surrounded by six ligand groups.

Step 2: (Step 2: Design a suitable ligand set)

To ensure the complex is chiral and does not contain any carbon atoms, we will need to select ligands that satisfy these conditions. Let's consider chlorine (Cl-), ammonia (NH3), and water (H2O) as potential ligands for our complex, as they do not contain carbon atoms.

Step 3: (Step 3: Apply the rules for chirality)

For our cobalt(III) complex to be chiral, it must have non-superimposable mirror images, or enantiomers, which can be achieved by having the central cobalt atom surrounded by four different ligands in an octahedral geometry. In this case, we can adopt the following arrangement for our ligand set: three different ligands (Cl-, NH3, and H2O) and an additional ligand that is a chiral ligand. For the chiral ligand, we can use an anionic bidentate ligand derived from NO-OH, called nitrito-N,ONO (charge of -1).

Step 4: (Step 4: Construct the chiral cobalt(III) complex)

Adopting the ligand set from step 3, we can now construct the chiral cobalt (III) complex: \[ \text{[Co(Cl)(NH}_3\text{)(H}_2\text{O)(NO-OH)]^2- \] This cobalt(III) complex will exist in two enantiomeric forms due to the tetrahedral arrangement of the four different ligands surrounding the cobalt(III) center, thus making it chiral. Furthermore, the complex does not contain any carbon atoms as required in the problem statement.