Neutron diffraction We used neutron diffraction data collected on gas-loaded Fe(d4-pz)[Pt(CN)4] to get insight into the configuration of the pyrazine moieties. The absence of the peaks characteristic of a perpendicular configuration (Figure ) is a strong indication to discard this possibility. Additionally, some peaks related to the parallel configuration show an increase (although this could be also in part correlated with the presence of gas molecules in the structure). Therefore we used the parallel configuration as starting model, which is consistent with DFT calculations (see below). In subsequent refinements, some disorder was allowed between the possible positions of the pyrazine. The residual scattering density obtained when the initial model of the bare Fe(pyrazine)[Pt(CN)4] with parallel pyrazines is compared to the experimental neutron diffraction pattern of the gas-loaded compounds (see Figure for the case of CO) allows us to distinguish two bonding sites for the guest molecules: (i) On top of the open-metal site (site A) and (ii) between the pyrazine rings (site B). The higher density is found in site A, which is an indication of a preference for this site. The residual density observed, located mainly in the z = 0 plane, is indicative of a position of the gas molecules in parallel to the Pt[CN]4 plane. The guest molecules were then incorporated to the model. Initially, these molecules were placed disordered in two perpendicular positions (see Figure ), and 2 and 1.5 molecules per f.u. were considered for CO and CO2, respectively (with site A always fully occupied, since it shows a significantly higher residual density). With this model, the disorder of the pyrazine rings between their two possible positions was estimated by a fit to the gas-loaded Fe(d4-pz)[Pt(CN)4] patterns, with refined values of 35(2) and 30(3) % for CO and CO2, respectively. This disorder was then fixed in the ensuing refinements. Finally, constrained fits (due to the quality of the data and the number of parameters) of the gasloaded Fe(pyrazine)[Pt(CN)4] patterns (see details in the SI) yielded the structural models presented in Figure . The structures present a slight monoclinic distorsion (of ca. 0.5 • in γ for both CO and CO2) , in agreement with DFT results (vide infra). The ordering of the pyrazine rings in a pref-erentially parallel configuration implies the existence of two non-equivalent B sites, one with more space available than the other. Attempts to introduce gas molecules in the site with less available space produced worse fits. A total occupancy of 2 CO molecules per f.u. gave the best agreement for the CO-loaded material, with both A and B sites fully occupied.