NON-FIRST-ORDER KINETICS OF THE HIGH SPIN-LOW SPIN RELAXATION IN FE(BPP)2(BF4)2 AFTER LIESST AND THERMAL SPIN TRAPPING

The iron(I1) spin crossover complex [Fe(bpp)2](BF4)2 (bpp = 2,6-bis(pyrazol-3-yl)pyridine) shows a complete high spin (HS, 5T2) =I low spin (LS, 'A') transition in the temperature region 160-185 K with a hysteresis of 10 K. It also shows the well-known LIESST and reverse LIESST effects at low temperatures. Trapping of the metastable HS state is also possible by rapid cooling of the sample. The HS LS relaxation was monitored at temperatures between 92.5 and 100 K using Mossbauer spectroscopy and shows strong deviations from firstorder kinetics, regardless of the origin of the metastable HS states. [Fe(bpp)*](BF& is the first spin crossover complex in which long-lived metastable HS states could be generated by LIESST at temperatures as high as 80 K.