Adsorption and activation of formaldehyde molecule by copper and silver sites in ZSM-5 was studied by combined QM/MM methodology. ETS-NOCV analysis was applied to decompose the deformation density (the difference between electron densities of composed systems and non-interacting fragments) into independent density flow channels. The analysis disclosed that the bonding of formaldehyde by the metal site may be described in terms of lone pair donation from carbonyl oxygen to the metal and pi*-backdonation from metal d orbitals to antibonding orbital of CO. The former one is responsible for the formation of coordinate bond ("dative covalent") and the latter one for the activation of carbonyl bond. Cu(I) site is more efficient with respect to both processes than Ag(I) one. Zeolitic environment is shown to weaken the bond between formaldehyde and M(I) with respect to free cations for both metals whereas it enhances pi*-backdonation in the case of copper and diminishes pi*-backdonation in the case of silver. Calculated red-shifts of CO frequencies agree well with the IR-measured ones and correlate with the degree of pi*-backdonation which rationalizes as well very good performance of Cu(I) site compared to Ag(I) as the role of zeolitic framework in modifying exchanged cation properties. (C) 2010 Elsevier B.V. All rights reserved.