Stereoisomerism in Complexes

Learning Objectives (OCR Spec 5.3.1)

By the end of this lesson you should be able to:

• Define stereoisomerism, cis-trans isomerism, and optical isomerism in the context of complex ions
• Draw cis and trans isomers of square planar and octahedral complexes
• Identify complexes that show optical isomerism (chirality)
• Explain how bidentate ligands (en, ethanedioate) give rise to optical isomers in octahedral complexes
• Describe the structure of EDTA4- as a hexadentate ligand

Revision: What is Stereoisomerism?

Stereoisomers are molecules with the same structural formula (same atoms, same bonds) but a different arrangement in space. You met stereoisomerism for organic compounds in Year 12 (E/Z isomers of alkenes, optical isomers of amino acids). In transition metal chemistry the same two types apply:

• Cis-trans (geometric) isomerism: the ligands can be arranged with similar ligands adjacent (cis) or opposite (trans).
• Optical isomerism: the complex has a non-superimposable mirror image (it is chiral). The two mirror-image forms are called enantiomers.

Cis-Trans in Square Planar Complexes

A square planar complex with the formula [Ma2b2] (two ligands a and two ligands b) can exist as cis (the two a's adjacent, 90 degrees apart) or trans (the two a's opposite, 180 degrees apart).

The classic example is cisplatin / transplatin, [Pt(NH3)2Cl2]:

  cis-[Pt(NH3)2Cl2]:        trans-[Pt(NH3)2Cl2]:
         Cl   NH3                Cl    NH3
          \ /                     \  /
           Pt                      Pt
          / \                     /  \
         Cl   NH3                H3N   Cl

• cis: two chlorides on the same edge - this is the active anti-cancer drug.
• trans: two chlorides diagonally across - this is clinically inactive.

Square planar [Pt(NH3)2Cl2] is the defining A-Level example of cis-trans isomerism in a complex. You must be able to draw both forms clearly with wedge bonds or equivalent.

Cis-Trans in Octahedral Complexes

An octahedral complex with the formula [Ma4b2] (four a and two b) can also show cis-trans isomerism:

• cis: the two b's are adjacent (at 90 degrees)
• trans: the two b's are on opposite sides (at 180 degrees)

Example: [Co(NH3)4Cl2]+, which can be drawn as:

  cis-[Co(NH3)4Cl2]+:         trans-[Co(NH3)4Cl2]+:
        NH3                         Cl
         |                           |
    Cl   |  NH3                NH3   |  NH3
      \  | /                     \  | /
       Co                          Co
      /  | \                     /  | \
    Cl   |  NH3                NH3   |  NH3
         |                           |
        NH3                         Cl

• cis: two Cl- adjacent (at 90 degrees), violet colour.
• trans: two Cl- opposite (at 180 degrees), green colour.

The two forms have different physical and chemical properties - they absorb light differently, have different solubilities, and in some cases different reactivities (similar to cisplatin).

Optical Isomerism

An object is chiral if it has a non-superimposable mirror image. Your two hands are chiral - you cannot place your left hand on top of your right so that all fingers align. A chiral complex is one whose mirror image cannot be superimposed on the original by any rotation.

Chiral molecules exist as two enantiomers. They are identical in every respect except:

• They rotate plane-polarised light in opposite directions (one clockwise, one anticlockwise)
• They may have different biological activity

At A-Level you need to know that octahedral complexes with three bidentate ligands ([M(en)3]^n+, [M(C2O4)3]^n-) are chiral.

Why [M(en)3] Is Chiral

The three bidentate ligands wrap around the central metal ion in a propeller-like arrangement. Looking down one of the three-fold axes, the bidentate ligands spiral either clockwise or anticlockwise. The two arrangements (delta and lambda, or right-handed and left-handed) are non-superimposable mirror images.

graph LR
  A[Left handed<br/>lambda<br/>anticlockwise propeller] -->|mirror| B[Right handed<br/>delta<br/>clockwise propeller]
  A -.-> C{Non-superimposable<br/>enantiomers}
  B -.-> C

Examples of chiral octahedral complexes OCR expects you to recognise:

• [Ni(en)3]2+ - tris(1,2-diaminoethane)nickel(II)
• [Co(en)3]3+ - tris(1,2-diaminoethane)cobalt(III)
• [Cr(C2O4)3]3- - tris(ethanedioato)chromate(III)

The Bidentate Ligands: en and Ethanedioate

1,2-diaminoethane (ethylenediamine, en)

Structure: H2N-CH2-CH2-NH2. Both nitrogens have a lone pair available for donation to the metal. When both coordinate to the same metal, they form a five-membered chelate ring with the M (one M - one N - one C - one C - one N).

A complex with en is:

        N - CH2
       /      |
      M        CH2
       \      |
        N - (closed)

Ethanedioate (oxalate, C2O4^2-)