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

[EN] 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.Financial support from the Spanish Ministerio de Educacion y Ciencia (MEC) (CTQ 2007-64727) is acknowledged. A.B.G. thanks the Spanish MEC for a research contract (Programa Ramón y Cajal). 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