The original formulation covered only metal–alkene bonds but the model was expanded over time to cover systems like metal carbonyls (including ) where π backbonding is important.
Formulasi awal hanya melingkupi ikatan logam–alkena tetapi model tersebut diperluas dari waktu ke waktu untuk mencakup sistem seperti logam karbonil (termasuk ) di mana ikatan balik π sangat penting.