The design of iron(II) spin crossover (SCO) complexes which exhibit wide thermal hysteresis have attracted much attention lately, since their bistable nature could allow their use as molecular switches in new electronic devices. In this aim, it is crucial to understand the main factors which control the spin crossover phenomenon.   We were interested to investigate the cooperative effects in the [Fe(dpp)2(NCS)2]py (dpp = dipyrido[3,2-a:2,3,-c]phenazine and py = pyridine) compound (Figure 1a) described in 1998 [1] which presents a fairly abrupt spin transition at 123 K in the cooling mode and 163 K in the heating mode, with a resulting hysteresis of 40 K (Figure 1b). In this study we used high-pressure single-crystal X-ray diffraction and Raman spectroscopy to study the spin crossover (SCO) between high spin (HS) and low spin (LS) states in the mentioned above compound. A single geometrical mechanism is responsible not only for the strongest negative linear compression (NLC) behaviour yet observed in a molecular material, combined with negative thermal expansion (NTE) and extreme positive linear compressibility, but also for the high cooperativity of the SCO in [Fe(dpp)2(NCS)2]·py. The scissor-like motion of individual molecules is propagated through the lattice via the physical intercalation of ligands and is also responsible for suppressing the expected HS→LS SCO under pressure [2].