Stereoisomerism: optical isomers

  Optical isomerism is concerned with chirality, and some important terms relating to chiral complexes are defined in Box 19.2. The simplest case of optical isomerism among d-block complexes involves a metal ion surrounded by three didentate ligands, for example [Cr(acac)₃] or [Co(en(₃]³⁺‏ (fig. 1.1).

Fig. 1.1 The octahedral complex [Co(en)₃]³⁺‏ contains three didentate ligands and therefore possesses optical isomers, i.e. the isomers are non-superposable mirror images of each other.

    These are examples of tris-chelate complexes. Pairs of enantiomers such as Δ- and Λ-[Cr(acac)₃] or Δ- and Λ-[Co(en)₃]Cl₃ differ only in their action on polarized light. However, for ionic complexes such as [Co(en)₃]³⁺‏, there is the opportunity to form salts with a chiral counter-ion A^-. These salts now contain two different types of chirality: the Δ- or Λ- chirality at the metal centre and the (+‏) or (^_) chirality of the anion. Four combinations are possible of which the pair {Δ-(+‏)g and {Λ-(^_)} is enantiomeric as is the pair {Δ-(^_)} and {Λ-(+)} However, with a given anion chirality, the pair of salts {fΔ-(^_)} and {Λ-(^_)} are diastereomers (see Box 19.2) and may differ in the packing of the ions in the solid state, and separation by fractional crystallization is often possible. Bis-chelate octahedral complexes such as [Co(en)₂Cl₂]‏⁺ exist in both cis- and trans-isomers depending on the arrangement of the chloro ligands. In addition, the cis-isomer (but not the trans) possesses optical isomers. The first purely inorganic complex to be resolved into its optical isomers was [CoL₃]⁶⁺‏ (1.1) in which each L‏ ligand is the complex cis-[Co(NH₃)₄(OH(₂]‏⁺ which chelates through the two Odonor atoms.

(1.1)

   Chirality is not usually associated with square planar complexes but there are some special cases where chirality is introduced as a result of, for example, steric interactions between two ligands. In 1.2, steric repulsions between the two R groups may cause the aromatic substituents to twist so that the plane of each C₆-ring is no longer orthogonal to the plane that defines the square planar environment around M. 

    Such a twist is defined by the torsion angle A–B–C–D, and renders the molecule chiral. The chirality can be recognized in terms of a handedness, as in a helix, and the terms P and M (see Box 19.2) can be used to distinguish between related chiralmolecules. If, in 1.2, the Cahn–Ingold–Prelog priority of R is higher than R’ (e.g. R = Me, R’ = H), then a positive torsion angle corresponds to P-chirality. An example is trans- [PdCl₂(2-Mepy)₂] (2-Mepy = 2-methylpyridine), for which the P-isomer shown in Figure 1.2.

(1.2)

Fig. 1.2 Two views of the structure (X-ray diffraction) of trans-[PdCl₂(2-Mepy)₂] (2-Mepy = 2-methylpyridine) showing the square planar environment of the Pd(II) centre and the mutual twisting of the 2-methylpyridine ligands. The torsion angle between the rings is 18.68 [M.C. Biagini (1999) J. Chem. Soc., Dalton Trans., p. 1575]. Colour code: Pd, yellow; N, blue; Cl, green; C, grey; H, white.