A synergic approach combining molecular dynamics (MD) and extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to investigate weak-concentrated (0.1 M) acetonitrile solutions of La(Tf2N)3 and Dy(Tf2N)3 salts (where Tf2N is the bis(trifluoromethanesulfonyl)imide). The MD simulations show that contact ion pairs between the Ln3+ cations and the Tf2N- anions are formed in the solutions. This finding has been experimentally confirmed by the analysis of the Ln K-edge EXAFS experimental signals of the two solutions. Both La3+ and Dy3+ ions preferentially form a 10-fold first shell complex composed of acetonitrile molecules and Tf2N- counterions with a bicapped square antisprism (BSAP) geometry. As a consequence of lanthanide contraction, the Dy3+ cation binds the inner shell solvent molecules at shorter distances as compared to La3+ and the high charge density of Dy3+ allows the coordination with additional ligands at longer distances. On the other hand, the bigger La3+ ion forms a very crowded coordination shell with a larger average distance and with the capped molecules at distances from the ion more similar to the inner shell ones. This peculiar coordination structure could explain the high catalytic activity of the Ln-Tf2N complexes and the high Lewis acidity of the lanthanide center.