A reversible or irreversible reaction that involves the introduction of a guest species into a host structure and that results in significant structural modifications to the host, for example, the breakage of bonds. Example: The insertion of lithium at 50°C into the spinel Li[Mn₂]O₄ with symmetry F d3m to yield a layered structure with symmetry P 3 ̄ m1. Alternative terms: topotactic reaction, insertion reaction.