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(II) spin crossover complex [Fe(bpp)(2)](BF4)(2) (bpp = 2,6-bis(pyrazol-3-yl)pyridine) shows a complete high spin (HS, T-5(2)) reversible arrow low spin (LS, (1)A(1)) 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 first-order kinetics, regardless of the origin of the metastable HS states. [Fe(bpp)(2)](BF4)2 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.