AbstractThermally‐induced spin variation behavior of [FeII(BPMEN)(CH3CN)2](ClO4)2 in CD3CN solution [low spin (LS)−high spin (HS)] was investigated. Variable temperature solution magnetic susceptibility measurements gave ΔH°LS−HS = 39.7 ± 2.0 kJ·mol−1 and ΔS°LS−HS = 135 ± 9 J·mol−1 K−1 values. These values are too high for a pure spin transition and reflect a significant contribution of chemical processes to the spin transition, namely, the detected dissociation of one bound acetonitrile molecule. Thus, LS−HS conversion should be regarded as a chemical equilibrium overlapped with a thermal spin crossover. The thermodynamic parameters for the overall process estimated from the variation of the NMR shifts with temperature (ΔH° = 40 ± 3 kJ·mol−1, ΔS° = 137 ± 10 J·mol−1 K−1) are in perfect agreement with those obtained by magnetic susceptibility measurements. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)