The chemisorption of atomic nitrogen on the four-fold site Cu(100) and Ag(100) surfaces has been modeled by Cu5N, AgsN and Cu41N cluster models. For these clusters, ab initio Hartree Fock electronic wave functions have been obtained and the chemisorption bond analyzed using a variety of techniques. In particular, the interaction energy has been decomposed by means of the constrained space orbital variation (CSOV), method. To obtain a reliable description of the interaction energy, electronic correlation effects have been explicitly accounted for. It is concluded that, contrary to other adsorbates such as halides or oxygen, the bond is essentially covalent but with a large charge transfer from the substrate to the adsorbate. Use of the CSOV analysis permits us to conclude that the direct covalent participation of the metal d electrons to the bond is very important. Finally, a preliminary attempt to study the relaxation of the substrate has been performed for the larger cluster only.