[Co-II(4-terpyridone)(2)]X-2: 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.