In this work we demonstrate the influence of an impurity atom in tuning the stability of Pb-13 cluster. For this purpose we have investigated the electronic and geometric structure of MPb12 (M = Pb, C, Al, Mg) clusters using the ab initio molecular dynamics simulation method. The energy gap between the highest occupied and lowest unoccupied energy levels of Pb-13 and MgPb12 is estimated to be 0.95 and 2.3 eV, respectively. The significant increase in the energy gap is resulted from the energy level reordering of Pb-13 by the incorporation of Mg atom. Further investigations of the stability of MPb12 clusters reveal that the interplay between the atomic and electronic structure is crucial to understand the stability of small size clusters. (c) 2006 Elsevier B.V. All rights reserved.