With the aim to rationalize the reactivity of transition metal (TM) clusters, the model reaction of NO dissociation on TM 13 clusters was investigated on a set of seven TMs. The adsorption energy and the activation energy for NO dissociation were calculated for each TM and discussed within the findings of Boudart in his article "Model catalysts: reductionism for understanding". In this work we focused on selected TM cluster calculations instead of surface science slab-type calculations, which have been performed for some TMs and focus on the electronic structure to investigate and rationalize the TM reactivity. Besides the electronic and spin properties of the TM we discuss the reactivity trends using also qualitatively the chemical hardness and softness. We conclude that the combination of these reactivity parameters will be necessary to fine tune TM clusters reactivity with the scope of multi-metallic TM clusters.