The second example of spin conversion governed by molecular vibrations:: a novel ferrous complex resulting from hexacoordination of a Schiff base with an N4O2 donor set

The crystal structure of [Fe-II(3-MeO,5-NO2-sal-N(1,10)-NMe(4,7))] (2) is identical to that of [Fe-II(5-NO2-sal-N(1,4,7,10))] (1), except for substitution of two strong N-H ... O(nitro) bonds (1) by one weak N-C-H ... O(nitro) contact (2) between adjacent molecules along the 1D chains. Unlike 1 (two-step spin-state transition), 2 experiences an incomplete (17%) and progressive (70-240 K) spin-state conversion (Mossbauer and magnetic studies). A theoretical approach based on the Ising-like model, completed with harmonic oscillators associated with the 15 vibrational modes of the coordination octahedron, fits the thermal variation of the high-spin (HS) fraction with a 2303 K energy gap between the lowest low spin (LS) and HS electrovibrational states, an average vibrational frequency omega(LS) = 725 K and an average omega(LS)/omega(HS) = 1.724 ratio. These results illustrate the effect of intramolecular vibrations spin-state conversions. (C) 1998 Elsevier Science B.V.