Comprehensive SummaryTo better understand the impact of different anions on the structures and SCO properties of the CoII SCO complexes, six new complexes [Co(terpyâCH2OH)2]A2·sol (terpyâCH2OH = 4âČâ(hydroxymethyl)â2,2âČ;6âČ,2âłâterpyridine, A = Brâ (1, sol = 1.5H2O), Iâ (2), N3â (3, sol = 2H2O), H3BCNâ (4), OTfâ (5), and TsOâ (6, sol = 4H2O·CH3CN), have been synthesized and characterized. All six compounds consist of mononuclear [Co(terpyâCH2OH)2]2+ cations and chargeâbalancing anions that differ in size, shape, and hydrogen bonding capacity. Complexes 1, 2, 3, and 6 displayed incomplete gradual SCO transitions, whereas 4 and 5 exhibited abrupt hysteretic spin transitions with loops of 12 and 16 K (250.0â262.0 K for 4, and 370.0â386.0 K for 5, respectively), closely resembling our previously reported complexes with SCNâ and SeCNâ anions. The occurrence of the orderâdisorder transition of the CH2OH groups and their transition temperatures are determined by the size and hydrogen bonding capability of the anions. Remarkably, the transition temperatures of complexes with H3BCNâ, SCNâ, OTfâ, and SeCNâ anions exhibit an upward trend as the size and mass of the anions increase, as confirmed through detailed single crystal structure analyses conducted in both highâspin and lowâspin states for all four complexes.