"However, Hahn and le Noble refute this point by invoking the principle of microscopic reversibility, where the process of bond formation and cleavage are fundamentally equivalent in an equilibrium, and little value should be placed on the terms ‘bonding’ and ‘antibonding’, σ or σ*.In another criticism of the model, Houk questions Cieplak's fundamental assumption that C-H bonds are better electron-donors than C-C bonds.