We have applied a phenomenological two-level Ising-like model to study the case of 3D coordination polymers showing strongly cooperative but incomplete spin-transition behaviour. Our numerical results focus on nanoparticles of the cyanide-bridged 3D network Fe(pyrazine)[Pt(CN)4], considering several factors (i.e. temperature, pressure), the role of short- and long-range interactions and the system size. To be consistent with the experimental studies on a nanometric scale, we used a semiexact method (Monte Carlo entropic sampling) to solve a system that contains molecules on each surface in the high-spin state (edge atoms). A preliminary analysis of the short-range antiferromagnetic case has been carried out to illustrate the three-step transition case.