Isomerism in transition metal complexes

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Structural isomers

These are compounds with the same number and type of atoms but the atoms are bonded in different arrangements. Ionisation isomers occur when the ligands forming coordinate bonds to a central metal ion in a complex swap places with free ions in solution.

E.g. [CrCl(NH3)5]SO4 and [Cr(SO4)(NH3)5]Cl

E.g. CrCl3•6H2O exists as three isomers with water molecules either coordinating to the Cr3+ ion as ligands or present as water of crystallisation. The chloride ions may also act as ligands within the complex or simply be anions that make up the salt.

Geometric / cis-trans isomers (stereoisomers)

We can find examples of geometric isomerism in tetrahedral, square planar and octahedral complexes, and in complexes with both monodentate and bidentate ligands.

Cis-trans isomers have varying chemical properties as a result of their differing geometries.

Cis-[PtCl2(NH3)2] is known as cis-platin and is an extremely effective anti-cancer drug, discovered by accident by the American scientist Barnett Rosenberg who was studying how electric fields might affect cell division. The platinum electrodes he was using formed small amounts of the platinum complex with the ammonium chloride buffer. Trans-[PtCl2(NH3)2] is biologically inactive.

The chloride ions are labile and easily replaced with other ligands but the concentration of chloride ions in the body is high enough to ensure that cis-platin reaches the cancer cells intact. Once the drug has crossed the cell membrane, the chloride ion concentration is lower and the chloride ions are replaced with water ligands.

cis-[PtCl2(NH3)2](aq) + 2H2O(l) ⇾ [Pt(NH3)2(H2O)2]2+(aq) + 2Cl–(aq)

The complex can now form a chelate ring with DNA (the central platinum ion accepts dative covalent / coordinate bonds from nitrogen atoms in DNA which replace the water ligands) disrupting the ability of the DNA to replicate and the cell to divide.

However, cis-platin only works on some tumours and is toxic as it targets all cells in the body, not just cancer tumours.

Other examples of cis-trans isomers you should be able to recognise are octahedral complexes …

and those with bidentate ligands …

Optical isomers (stereoisomers)

Optical isomers have a chiral centre and are asymmetric which means that the mirror image of one isomer is non-superimposable on the other.

We see this in tetrahedral complexes where the central metal ion is bonded to 4 different ligands such as in [CoBrClFI]2-. It is difficult to isolate one isomer from another because in reality these ligands readily move around (they are labile) and one isomer is rapidly converted into the other in a process known as racemization.

Octahedral complexes formed when three bidentate ligands coordinate to a central metal ion exist as pairs of optical isomers (also called enantiomers). Bidentate ligands are far less labile than monodentate ligands so it is possible to isolate one optical isomer from the other.

And finally, an octahedral complex with two bidentate ligands and two monodentate ligands exists as a pair of optical isomers in the cis geometry but not in the trans geometry (where the mirror images are superimposable).

Practice questions

  1. (a) Suggest a chemical test to distinguish between the ionisation isomers [CrCl(NH3)5]SO4 and [Cr(SO4)(NH3)5]Cl.

(b) Suggest a chemical test to distinguish between the chromium salts [Cr(H2O)6]Cl3 and [CrCl(H2O)5]Cl2•H2O.

  1. Fe3+ forms a complex with two ethanedioate ligands, C2O42-, and two water molecules. The complex ion exists as a cis-isomer and a trans-isomer. One of these isomers also exists as optical isomers.

(a) Determine the formula of the complex ion.

(b) Draw the 3-D structure of each of the stereoisomers.

  1. Cis-platin, [PtCl2(NH3)2], is a complex that has been found to be effective in fighting cancer tumours. When it enters a cell one of the chloride ions is replaced by a water molecule, forming complex ion X.

(a) Write an equation for this reaction.

(b) Complex ion X is able to bridge two guanine nucleotides between strands of DNA. Both the water and the chloride ligands are replaced with coordinate bonds between the Pt2+ and a nitrogen atom in each guanine nucleotide. Show how this bridge forms on the diagram below.

Answers

  1. (a) Add aqueous silver nitrate solution to a sample of each isomer. Ag+ will react with the free chloride ions in [Cr(SO4)(NH3)5]Cl and a cloudy white precipitate will be observed. There will be no reaction with [CrCl(NH3)5]SO4 as the chloride ion is bonded to the Cr in the complex ion and not free to react.

(b) [Cr(H2O)6]Cl3 has 3 free chloride ions per complex and [CrCl(H2O)5]Cl2•H2O has two. If a known mass of each salt is dissolved in water and then excess silver nitrate solution added, the precipitate formed could be filtered, dried and weighed. [Cr(H2O)6]Cl3 would give a higher mass of precipitate.

  1. (a) [Fe(C2O4)2(H2O)2]–

(b)

  1. (a) [PtCl2(NH3)2] + H2O ⇾ [PtCl(NH3)2(H2O)]+ + Cl–

(b)