3. Isomerism in Transition metal complexes

Case studies: 3.1 possibilities * 3.2 cis-platin * 3.3 ionisation isomerism

3.4 stereoisomerism in Co³⁺ complexes * 3.5 co-ordination isomerism

Case study 3.1 A general survey of some possible situations

The spatial arrangement of the ligands in complex ions can allow the existence of isomers. (1) and (2) show two examples of how cis/trans geometrical isomers can exist with two different ligands (shown as red L's and blue L's) for co-ordination numbers 4 (square planar) and 6 (octahedral). (3) shows how optical isomers (enantiomers) can exist when three bidentate ligands are bonded to a central metal ion.

Just as in the behaviour of natural or synthesised optically organic molecules like amino-acids or drugs, the stereochemistry can affect the pharmacological behaviour of inorganic compounds too.

Case study 3.2 Geometrical isomerism in square planar platinum complexes

e.g. cis-diamminedichloroplatin(II), [Pt(NH₃)₂Cl₂]⁰

Known as cis-platin, the above complex is one of the most effective agents against cancers of the ovaries, bladder, and head and neck and helps as co-agent in the treatment of cancers of the cervix, lung and breast. Its biggest success has been in the treatment of testicular cancer, a form of cancer previously resistant to any therapy but now considered to be curable in most cases. The platinum stereochemistry is very important, with the trans isomer showing no anti-cancer activity compared to the cis-isomer which is pharmacologically active (diagram below).

based on [Pt(NH₃)₂Cl₂]

cis/trans-platin are neutral, square planer complexes of co-ordination number 4, exhibiting geometrical isomerism. There are two of each of the monodentate ligands, namely the chloride ion and ammonia and the blue lines/dots show the 4 dative covalent bonds.

Case study 3.3 Ionisation/hydrate isomerism in Transition metal complexes

e.g. Four complexes of chromium(III) chlorides based on a Cr³⁺, 3Cl^-'s and 6H₂O. Four different crystal forms can be made. The different colours arise from small differences in the electronic energy levels of the central transition metal ion caused by changes in the ligand arrangement.

(a) [Cr(H₂O)₆]³⁺(Cl^-)₃  (violet or grey-blue?),

(b) [CrCl(H₂O)₅]²⁺(Cl^-)₂.H₂O  (pale green)

(c) [CrCl₂(H₂O)₄]⁺Cl^-.2H₂O  (dark green, can also exist as cis or trans isomers - see case study 3.1 example 2 in diagram, L₄ and L₅ would be the two Cl ligands)

(d) [CrCl₃(H₂O)₃]⁰.3H₂O (brown?), the ⁰ can be omitted, it just signifies an overall electrically neutral complex, (a) to (c) are complex ions.

Case study 3.4 Examples of optical isomerism

Cobalt octahedral complexes. e.g.

[Co(H₂NCH₂CH₂NH₂)₃]³⁺ consists of a Co³⁺ ion combined with three bidentate ligands (1,2-diaminoethane, dative bond N:=>Co, via the lone pair of electrons on the nitrogen). See (3) in case study 3.1 above.

[Co(H₂NCH₂CH₂NH₂)₂NH₃Cl]²⁺ consists of a Co³⁺ ion combined with two 1,2-diaminoethane bidentate ligands, one chloride ion and one ammonia molecule.

[Co(H₂NCH₂CH₂NH₂)₂Cl₂]⁺ consists of a Co³⁺ ion combined with two 1,2-diaminoethane bidentate ligands and two chloride ions.

All three of these can exist as 'mirror image' forms, i.e. enantiomers.

Case study 3.5 Co-ordination isomerism

[Co(NH₃)₆]³⁺[Cr(C₂O₄)₃]^3- or [Co(C₂O₄)₃]^3-[Cr(NH₃)₆]³⁺ 

Here in this Co³⁺/Cr³⁺ anionic-cationic complex the ligands attached to the central metal ion can be exchanged.

The two ligands involved are the electrically neutral ammonia molecule, :NH₃, and the ethanedioate anion (2-), ^-OOC-COO^- derived from the dicarboxylic acid, ethandioc acid.