This paper is concerned with the first two-step spin crossover presented by a polynuclear molecular compound, viz., the dinuclear iron(II) complex [Fe(bt)(NCS)2]2bpym, where bt stands for 2,2'-bi-2-thiazoline and bpym for the bridging ligand 2,2'-bipyrimidine. The synthesis of the compound is described. Variable-temperature magnetic susceptibility and 57Fe Mossbauer spectrometry data provide evidence for an overall 5 = 2 (HS) ** S = 0 (LS) spin-crossover behavior. They show that the transition takes place in two steps. The so-called "step 1" and "step 2" are centered around 163 and 197 K, respectively. The former is more abrupt than the latter. The thermal variation of the quadrupole splitting of the HS doublet (A£qHS) confirms that the sample is made of a single structural phase and roughly accounts for the intramolecular processes LS,LS ** LS,HS (step 1) and LS,HS ** HS,HS (step 2). The differential scanning calorimetry diagram exhibits two peaks, a sharp one pointing at 164 K and a broad one with a maximum at 194 K. The overall enthalpy and entropy variations are found to be AH = 13.3 ± 0.5 kj mol™1 11and AS = 82 ± 6 J K™1 mol'1, respectively. The cornerstone of the theoretical approach which has been developed to account for a two-step transition in iron(II) dinuclear species is that the enthalpy HSq of the mixed-spin species SQ (S = spin singlet; Q = spin quintet) is not exactly halfway between the enthalpies Hss and Hqq of the like-spin species SS and QQ, respectively. It turns out that it can be written HSq = (Hss + Hqq)/2 + W. W ^0 is due to both electrostatic and vibronic effects. W > 0 makes the transition more abrupt; on the other hand, W < 0 may lead to a two-step transition.Actually, the two-step character is due to the synergistic effect of intramolecular interactions favoring SQ (W < 0) and intermolecular interactions favoring like-species domains (intermolecular interaction parameter y > 0). Our model leads to quite a satisfying fitting of the magnetic data with AH = 13.16 kJ mol™1, AS = 73.69 J K™1 mol™1, W = -478 J mol™1, and 7 = 2.572 kJ mol™1. Furthermore, using those energy parameters results in a simulation of the heat capacity versus temperature curve showing the main features of the experimental curve, in particular a sharp and intense peak around 163 K and a broader and less intense peak around 197 K.

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