Did you know that the haemoglobin in your blood, the chlorophyll in green plants and vitamin B-12 all contain compounds know as transition metal complexes [1]? Transition metal complexes, also known as coordination complexes, consist of ions or molecules coordinated to a central transition metal ion or atom. The ions or molecules that bind to the central metal ion or atom to form these complexes are called ligands. The number of ligands bound to the central metal ion or atom is called the coordination number.  

Transition metal complexes can have different shapes depending on its coordination number. The shapes that are common for transition metal complexes formed using monodentate ligands (ligands which only form one bond to the central metal ion or atom) are tetrahedral, square planar and octahedral [1], as shown below. 6-co-ordinated complex ions, in which the central metal is  attached to six ligands, have an octahedral shape [2]. On the other hand, 4-co-ordinated complex ions, in which the central metal is attached to four ligands, can either have a tetrahedral or square planar shape [2]. The difference between these 2 shapes lies in the bond angle— in tetrahedral structures, the bond angle is 109.5 degrees, while in square planar structures, the bond angle is 90 degrees.

Apart from organic compounds, transition metal complexes also exhibit stereoisomerism, namely geometric isomerism and optical isomerism. However, geometric isomers are only possible for square planar and octahedral complexes, but not tetrahedral [2]. Conversely, optical isomers are possible only for tetrahedral and octahedral complexes, but not square planar [2]. This is summarised in the flowchart below.

Metal complexes that differ only in which ligands are adjacent to one another (cis) or directly across from one another (trans) in the coordination sphere of the metal are called geometrical isomers [3]. They are most important for square planar and octahedral complexes. cis- and trans- refer to the position of 2 groups relative to each other. In the cis- isomer they are “next to each other”, at 90 degrees in relation to the central metal ion. On the other hand, in the trans- isomer they are “opposite each other”, at 180 degrees relative to the central metal ion. Because there is no way to convert the cis structure to the trans by rotating or flipping the molecule in space, they are fundamentally different arrangements of atoms in space [3].

One of the most well-known examples of cis and trans isomers of a square planar transition metal complex are cis-Pt(NH3)2Cl2, also known as cisplatin, and trans-Pt(NH3)2Cl2, also know as transplatin.

Cisplatin is an anticancer drug and is especially effective against tumours of the reproductive organs, which primarily affect individuals in their 20s and were notoriously difficult to cure. For example, after being diagnosed with metastasised testicular cancer in 1991 and given only a 50% chance of survival, Lance Armstrong was cured by treatment with cisplatin [3]. Conversely, its trans isomer, transplatin, is inactive. 

Optical Isomer 

Optical isomers are non-superimposable mirror images of each other that differ in the direction with which they rotate plane polarised light. An example of optical isomerism exhibited in Ru(en)32+ is shown below.

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References

1. 1. Rice University. Coordination Chemistry of Transition Metals. Available from: https://opentextbc.ca/chemistry/chapter/19-2-coordination-chemistry-of-transition-metals/.
   2. Lancashire, R.J. Isomerism in Coordination Compounds. 2015; Available from: http://wwwchem.uwimona.edu.jm/courses/IC10Kiso.html  
   3. Stereoisomers: Geometric Isomers in Transition Metal Complexes. 2017; Available from: https://chem.libretexts.org/Textbook_Maps/Inorganic_Chemistry/Supplemental_Modules_(Inorganic_Chemistry)/Coordination_Chemistry/Properties_of_Coordination_Compounds/Isomers/Stereoisomers%3A_Geometric_Isomers_in_Transition_Metal_Complexes.