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.)

Short answer

A chiral cobalt(III) complex without carbon atoms can be designed by using cobalt(III) and enantiomers of nitrogen-containing bidentate ligands without carbon, such as \(H_2NPH_2PH_2NH_2\). The resulting chiral cobalt(III) complex would have the formula \([Co(PH_2PH_2NH_2)_3]\), assuming a Δ or Λ configuration to achieve chirality.

Step-by-step solution

Understand the requirements of a chiral complex

To be chiral, a complex must have no planes of symmetry, which means it cannot be superimposable on its mirror image. In coordination chemistry, this generally means that the central metal ion is bonded to different ligands (or groups of ligands) in a non-symmetrical arrangement.

Choose a suitable central metal ion

As specified in the exercise, the central metal ion should be cobalt(III). Its oxidation state (III) allows it to form six-coordinate complexes with octahedral geometry, which can be chiral if the ligands are arranged appropriately.

Choose appropriate ligands without carbon atoms

To design a chiral cobalt(III) complex without carbon atoms, we need to choose ligands that are either chiral themselves or can create chirality when arranged around the central metal ion. In this case, since we cannot have carbon atoms, using enantiomers of nitrogen-containing bidentate ligands would be a suitable choice. A suitable ligand in this case would be ethylenediamine (\(H_2NCH_2CH_2NH_2\)), but replacing the carbon atoms with non-carbon atoms, in this case, phosphorus, we obtain \(H_2NPH_2PH_2NH_2\).

Combine the central metal ion and ligands in a chiral arrangement

Now we need to combine the cobalt(III) ion with the ligands in an octahedral arrangement. We will use three of the modified ethylenediamine ligands (\(H_2NPH_2PH_2NH_2\)) in a Δ or Λ configuration to create a chiral cobalt(III) complex.

Write the formula for the chiral cobalt(III) complex

Combining the cobalt(III) ion and the chosen ligands, we arrive at the following formula for the chiral cobalt(III) complex: \[ [Co(PH_2PH_2NH_2)_3] \] This complex meets the requirements of having no carbon atoms and being chiral if it could be synthesized.