The nature of the spin-state variation of [Fe-II(BPMEN)(CH3CN)(2)](ClO4)(2) in solution

Thermally-induced spin variation behavior of [Fe-II(BPMEN)(CH3CN)(2)](CIO4)(2) in CD3CN solution [low spin (LS)-high spin (HS)] was investigated. Variable temperature solution magnetic susceptibility measurements gave DeltaHdegrees(LS-HS) = 39.7 +/- 2.0 kJ(.)mol(-1) and DeltaSdegrees(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 (DeltaHdegrees = 40 +/- 3 kJ(.)mol(-1), DeltaSdegrees = 137 +/- 10 J(.)mol(-1) K-1) are in perfect agreement with those obtained by magnetic susceptibility measurements. (C) Wiley-VCH Verlag GmbH Co.