Spin-crossover (SCO) complexes exhibit cooperative spin transitions with changes in temperature, pressure, light, or other external stimuli. These transitions can lead to thermal hysteresis, which has enabled the development of SCO-based applications such as memory storage, spin switching, molecular electronics, and molecular sensors. In these systems, transition metal (TM) ions typically change from a high spin (HS) state at higher temperatures to a low spin (LS) state at lower temperatures, accompanied by a reduction in metal-ligand bond length. SCO is a collective property that affects many molecules within a sample over a wide temperature range, and sharper transitions are typically due to cooperative structural changes in the solid-state. The equilibrium populations of high spin (HS) and low spin (LS) state within a sample are equal at the transition temperature, or T1/2, for a spin-crossover (SCO) complex. The most extensively studied SCO complexes have been six-coordinate Fe(II) species, which undergo spin-state changes from S = 2 (HS) to S = 0 (LS) via a one-step mechanism or sometimes through an intermediate S = 1 state via a twostep mechanism. Other transition metal ions, such as Fe(III), Cr(II), Mn(III), and Co(II), are also known to exhibit SCO behavior. However, their properties and potential applications are not, in general, as well understood as those of Fe(II)-based complexes.