A sample comprising 241 π-complexes LnC n H m Z k (Z = O, S, Se, Te) was used to analyze the influence of the nature, the oxidation state, and the coordination number of the central atom and the nature of the Z atom on the activity area of the lanthanide (Ln). A new system of criteria based on the concepts of the atomic and molecular Voronoi–Dirichlet polyhedra is proposed for the quantitative analysis of intermolecular contacts. This system was used to analyze the influence of steric factors on the stability of complex groups and the presence or absence of agostic interactions in the structures of 475 complexes LnC n H m , LnC n H m X k (X = F, Cl, Br), and LnC n H m Z k .