In this talk, I would like to present the recent theoretical progress (within the shell model framework) in the description of semi magic nickel and tin isotopes. The role of the core excitations for the development of the quadrupole collectivity will be discussed in detail along the nickel chain, where novel SM calculations in an extended fpg9/2d5/2 model space will be presented. I will also show that these relevant degrees of freedom are of a crucial importance for the description of the new island of inversion below Ni68. Finally, I will discuss the underlying shell evolution and in particular I will focus on the rigidity of doubly-magic Ni78 and Sn100.