Isomerism in coordination compounds

[Big-Daddy]

Can someone outline a comprehensive way to figure out and write all the different isomers of square planar and octahedral single-centre coordination compounds, without needing to be able to see them in 3D (and without a computer)? Questions come up all the time on my exams and a proper way of being able to answer them systematically is what I want.

We can assume all ligands are monodentate for now, but I'd like a method which applies whatever the configuration of the compound is (e.g. one which applies for [Ma2b2] square planar compounds just as it does to [Mabcdef] octahedral ones).

 

[jbsteele]

To determine all the different isomers of square planar and octahedral single-centre coordination compounds without needing to be able to see them in 3D (and without a computer), one can use the following systematic approach: 1. For square planar compounds, first consider the four ligands coordinated to the central atom in two pairs of opposite faces. Each pair of ligands can be arranged in two distinct ways (clockwise or counter-clockwise). Therefore, there are a total of 2^2 = 4 possible isomers for square planar geometry. 2. For octahedral compounds, first consider the six ligands coordinated to the central atom in three pairs of opposite faces. Each pair of ligands can be arranged in two distinct ways (clockwise or counter-clockwise). Therefore, there are a total of 2^3 = 8 possible isomers for octahedral geometry. In addition to considering the arrangement of the ligands relative to the central atom, it is also important to consider the relative stereochemical configuration of the ligands themselves. If the ligands are chiral, then there will be additional isomers based on their configuration. For example, for an octahedral compound with three chiral ligands, there could potentially be a total of 8x2^3 = 64 possible isomers. If the ligands are achiral, then there will only be 8 isomers. It is also important to consider the possibility of different “conformers” of the same compound, which are different arrangements of the same ligands around the central atom. These conformers will have the same chemical formula but different geometries, and therefore will be counted as separate isomers. Finally, it is important to consider different tautomers of the same compound, which are different isomers that differ in the way the atoms are bonded together. These tautomers can be identified by examining the molecular structure and identifying any possible isomeric forms. By following these steps, it is possible to systematically determine all the different isomers of square planar and octahedral single-centre coordination compounds without needing to be able to see them in 3D (and without a computer).