Polymorphism and reverse'' spin transition in the spin crossover system [Co(4-terpyridone)2](CF3SO3)2•1H2O

Compound [Co(4-terpyridone)(2)](CF3SO3)(2)center dot 1H(2)O, where 4-terpyridone is 2,6-bis(2-pyridyl)-4(1H)-pyridone, forms two polymorphs. Polymorph 1 displays a continuous spin conversion in the temperature region 300-120 K while polymorph 2 shows, on cooling, the onset of a continuous high-spin (HS) to low-spin (LS) conversion interrupted by an abrupt reverse'' spin transition in the temperature region 217-203 K. The formed unstable HS intermediate phase (IP) undergoes a strong cooperative normal'' spin transition characterised by a hysteresis loop 33 K wide. The structural data give support for a crystallographic phase transition, which takes place concomitantly with the reverse'' spin state transition.