[CoII(4-terpyridone)2]X2: A Novel Cobalt(II) Spin Crossover System [4-Terpyridone = 2,6-Bis(2-pyridyl)-4(1H)-pyridone]

The cationic complex [Co(4-terpyridone)2]2+ where 4-terpyridone is the terpy-like ligand 2,6-bis(2-pyridyl)-4-(1H)-pyridone has been synthesized. High-spin and different spin crossover behaviors have been observed in the solid state depending on the counterion SO42-, Cl-, or ClO4-. The room temperature crystal structure of the spin crossover compound [Co(4-terpyridone)2](ClO4)2·H2O is described.