  Spin crossover frameworks and molecular crystals display fascinating
collective behaviours. This includes multi-step transitions with hysteresis and
a wide variety of long-range ordered patterns of high-spin and low-spin metal
centres. From both practical and fundamental perspectives it is important to
understand the mechanisms behind these collective behaviours. We study a simple
model of elastic interactions and identify thirty six different spin-state
ordered phases. We observe spin-state transitions with between one and eight
steps. These include both sharp transitions and crossovers, and both complete
and incomplete spin crossover. We demonstrate structure-property relationships
that explain these differences. These arise because through-bond interactions
are antiferroelastic (favour metal centres with different spin-states);
whereas, through-space interactions are typically ferroelastic (favour the same
spin-state). In general, rigid materials with longer range elastic interactions
lead to transitions with more steps and more diverse spin-state ordering, which
explains why both are prominent in frameworks.
