Neutron experiments were performed in the high-intensity two-axis diffractometer D20 installed on the hot neutron source of the high-flux reactor at the Institut-Laue Langevin (ILL) in Grenoble, France. A wavelength of 1.54 Å was used. The empty and gas-loaded samples were measured at 100 K. Rietveld refinements and calculations of the structures were performed using the FullProf suite of programs. The schematic illustrations of the crystal structures and magnetic arrangements were obtained with the VESTA program. Inelastic Neutron Scattering. Inelastic neutron scattering experiments were performed in the indirect geometrytype spectrometer IN1-LAGRANGE installed on the hot neutron source of the high-flux reactor at the Institut Laue Langevin (ILL) in Grenoble, France. Monochromators of Si(111), Si(311) and Cu(220) were selected to collect the data for energy transfers of [8-129], [92-215] and [173-427] cm -1 , respectively. The measurements were done at 30 K for both the empty and loaded materials. A post-processing treatment of normalization to monitor counts and subtraction of the empty sample holder was done using LAMP. Gas adsorption The powder samples were placed inside a cylindrical aluminium sample holder connected to a sample stick adapted for gas adsorption. A manifold gas pumping system was attached to the stick through a capillary and the temperature control was achieved using either a closed-cycle cryostat (IN1-LAGRANGE) or an Orange cryostat (D20). The samples were loaded to saturation by initially injecting the gas doses at room temperature and then lowering the temperature of the sample holder (to 100 K for CO and 200 K for CO2), creating a cold point to force the gas to move towards the sample and facilitate the adsorption. For CO2, the saturation loading is about 1.5 mol of CO2 per Fe mol, as determined experimentally from adsorption isotherms. In the absence, to the best of our knowledge, of published absorption isotherms for CO, we estimate the saturation loading at about 2 CO mol / Fe mol from the kinetic uptake experiments reported by Ibarra et al. Our ND, DFT and INS results are consistent with these saturation loading values (vide infra).