The Nature of the Spin‐State Variation of [Fe II (BPMEN)(CH 3 CN) 2 ](ClO 4 ) 2 in Solution

Thermally‐induced spin variation behavior of [Fe II (BPMEN)(CH 3 CN) 2 ](ClO 4 ) 2 in CD 3 CN solution [low spin (LS)−high spin (HS)] was investigated. Variable temperature solution magnetic susceptibility measurements gave Δ H ° LS−HS = 39.7 ± 2.0 kJ·mol −1 and Δ S ° 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 (Δ H ° = 40 ± 3 kJ·mol −1 , Δ S ° = 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)