AbstractA review of an extensive study on the preparation and solid‐state reactivity of squareplanar NiX2L2complexes is presented, where X = Cl, Br, I, or NCS and L = P(CH2CH2CN)3. Physical, chemical, and crystallographic criteria have been evaluated for such complexes, from the viewpoints of the properties and reactivity of the squareplanar monomer and the analogous octahedral polymer. Factors such as nearest‐neighbor interactions, nucleation, temperature, doping, polymorphism, and crystal symmetry greatly influence the course of the thermal solid‐state reactions. The reactionsper seare characterized by chemical and crystallographic specificity, stereodirectionality, highly anisotropic front motion in single crystals, and reversibility. In a series of preliminary, related experiments, the radiation‐induced polymerization of salts and complexes of β‐acryloxypropionic acid and propiolic acid has been studied; there is a great variation in reactivity among these species. Many of the reactive phases have short contacts between neighboring molecules which would appear to satisfy topochemical criteria for reaction in the s