We apply a model recently proposed for the analysis of the spin transitions in molecular crystals of the spin active complexes of Fe(II) to analysis of some experimental data. The molecules undergoing the spin transition are modeled by octahedra of variable size which interact when they are immediate neighbors in the crystal lattice. We show that if the crystal geometry is allowed to relax in the course of the transition the cooperative sign of the effective intermolecular interaction appears and thus the spin transition becomes a first order phase transition with the finite change of the fraction of the high spin molecules as a function of temperature. The magnitude of the effective interaction is estimated from the data on the crystal compressibility and within the assumption of the van der Waals character of the forces responsible for the effective interaction.