Two main mechanisms for expanding main group element valence shells.

  1. Classic (octet-expansion), using not only d, but higher s and p (Rydberg) shells.
  2. A non-Rydberg approach: Occupancy of non- or anti-bonding levels formed from non-Rydberg valence AOs.

    For more detail, see dipc.ehu.es/bondslam/index.php/Hypervalence

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