Unifying pictures of chemical bonding


As I have posted before I always like papers by Roald Hoffmann for their clarity and chemical intuition. The latest one I have read is Bonding in the trihalides (X3–), mixed trihalides (X2Y–) and hydrogen bihalides (X2H–). The connection between hypervalent, electron-rich three-center, donor–acceptor and strong hydrogen bonding
It seems the concept of four electron-three orbital bonding has wide chemical applications. I also noticed that the Figure below is reminiscent of the "backbonding" which occurs in organo-metallic complexes.

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