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Several research papers and books say that the meridional isomer of tris-(8-hydroxyquinoline) aluminium is thermodynamically more stable than the facial isomer.

What is the reason behind this stability?

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The stability of meridional isomer over facial isomer can be deduced by considering the interactions between atoms of the ligand 8-hydroxyquinoline. In considering the interactions if we take the repulsion between the electron rich atoms dominated in 90 degrees and at 180 degrees these interactions become negligible, we will find that in meridional isomer, there are dominating i) 2 N-N interactions ii) 2 O-O interactions iii) 8 N-O interactions. But if we look into the facial isomer of tris(8-hydroxyquinolinato)aluminium we will find there are dominating i) 3 N-N interactions ii) 3 O-O interactions iii) 6 N-O interactions. So in facial isomer two N-O interactions of mer isomer has turned into 1 N-N interaction and one O-O interaction. Due to high charge density on small sized O atoms O-O repulsions are much much greater than N-O interactions and thus the electronic repulsions are much more greater in fac isomer which decreases its stability than meridional isomer

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