As noted above, between 150 K and 25 K, the magnetic susceptibility decreases rapidly, indicating changes in the spin states of the dimeric Cr(II) species. There is also a corresponding appearance in the XRD experiments of two distinct Cr(II)-dimeric units, cations I and II, with the former exhibiting an increasingly short Cr-Cr internuclear distance as the temperature is lowered, while the latter Cr-Cr distance remains reasonably constant (although at 12 K, both cation geometries have short Cr-Cr distances of ~2. As temperature decreases, and the Cr-Cr distance shrinks, the triplet state of cation I becomes increasingly stable relative to the quintet state. Between 𝑋 130-I and 𝑋 110-I , the triplet crosses the quintet to become the ground state for cation I. The singlet state energy remains roughly constant relative to the ground spin state down to 𝑋 25-I and then at 𝑋 12-I , the singlet becomes the ground state as the Cr-Cr internuclear distance for cation I drops from approximately 2.9 Å to 2.8 Å. Over that last change in temperature, the triplet and quintet spin states are each destabilized by about 10,000 cm -1 relative to the singlet. Cation II exhibits a simpler temperature-dependent spin-state energy ladder as its relatively constant Cr-Cr internuclear distance (down to 50 K) favors the quintet state with a quintet-triplet gap of about 2000 cm -1 at all temperatures (see Figure ). At 𝑋 25-II , as the Cr-Cr distance in cation II begins to shrink, the quintet-triplet gap falls below 1400 cm -1 , and at 𝑋 12-II , the structure of cation II becomes similar to cation I and exhibits a singlet ground state.