As a rule, rational design of cooperative spin-crossover (SCO) molecular switches is largely based on consideration of sizes and structures of individual building blocks, whereas a meticulous analysis of crystal packing, including the weakest intermolecular interactions, is often assumed to play a secondary role or is even fully neglected. By investigating cobalt(II) clathrochelates, which do not change the molecular volume upon SCO, we showed that even weak (1.2 kcal/mol) π···Cl intermolecular interactions can cause a pronounced anticooperativity of SCO, being more gradual in the solid state than in solution. Our results clearly demonstrate that the "chemical pressure" concept is not as general as it is thought to be, and the successful design of molecular switches requires in-depth analysis of intermolecular interactions, however weak they seem. SECTION: Molecular Structure, Quantum Chemistry, and General Theory S pin-crossover (SCO) complexes 1,2 are very popular in the area of molecular electronics 3−5 owing to the long-recognized possibility of using the energy difference between low-spin (LS) and high-spin (HS) states of a transition-metal ion to create a molecular switch. 6,7 As in most cases, temperature-induced SCO has an intrinsically statistical nature, it is hard to obtain a purely two-state molecular switch. 8 In solids, however, the cooperative effects often lead to the hysteresis in SCO curves, allowing discrimination between purely LS and HS states. 9 It is generally accepted now that the cooperativity in SCO is a result of elastic interactions, 10