Bohr (1922, dd3mmx), Langmuir (1919-1921, bzg77j), Bury (1921, dmv98x)

• Introduce the 2, 8, 18, 32 filled shell rules:
  • Applied to the transition elements and beyond
  • Placed on a firm theoretical foundation between 1928-33
• Hypervalence ≡ extending the 8 electron rule for main group elements using partially filled d-shells.
  • Main group Octet-expansion ≡ Main group Hypervalence
  • ∴ Main group valencies > 4?^‡

By the 1950s, most text books adopt the convention of representing higher coordination main group compounds invoking (by implication) the Lewis two-electron bond and hence (by implication) utilising the d-shells. "Octet-expansion" in main group molecules still being taught in this manner.

FAIR Data doi: cgxv ^‡Question: what are the highest bond indices for transition elements with filled 18 electron shells? 9? (DOI: ck34)