It is considered that the reason why cyclometalation reactions for producing organometallic intramolecular-coordination five-membered ring compounds proceed extremely easily is that the reactions first proceed through metal activation initiated by the coordination of lone electron pair, such as N, P, O, or S, to a metal atom. This is followed by γ-C–H agostic interactions, C–H activation, and the chelate effect in this order (Eq. (6.7)). A recent report entitled “Chelation-Assisted Reactions (Eqs. (6.42)–(6.51))” provides evidence that the chelate effect is a strong activation source for cyclometalation reactions.