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 $50{}^{\text{⚬}}\text{C}$ into the spinel Li[Mn₂]O₄ with symmetry $F\text{d3m}$ to yield a layered structure with symmetry $P\overline{3}\text{m1}$. Alternative terms: topotactic reaction, insertion reaction.