Monday, 12 November 2018

Electron donating and withdrawing groups

Aside from the fact that I do not believe in the existence of HOMOs and LUMOs, it is sometimes good to know how they work. In particular, I can never remember how electron-donating and withdrawing groups work. Here is how I understand it:

  • An electron-donating group adds more electrons to the system and thus increases electron-electron repulsion (or decreases the effective nuclear charge). As a consequence the HOMO and LUMO energies increase.
  • An electron-withdrawing group removes electrons and, thus decreases the HOMO and LUMO energies.
  • An electron-donating group usually acts through an occupied non-bonding orbital. This is energetically close to the HOMO. Therefore, it has a stronger effect on the HOMO than on the LUMO (at least in organic molecules).
  • An electron-withdrawing group acts through a virtual orbital, which interacts more strongly with the LUMO.
  • As a consequence, electron-donating and withdrawing groups are both expected to lower the HOMO-LUMO gap in organic molecules.
  • Things are different for transition metal complexes. For example an electron-withdrawing fluorine group still lowers orbital energies. But it can affect the HOMO more strongly and increase the overall gap in fluorinated iridium complexes, see this Ref.

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