An investigation of 135 p-complexes of rare-earth atoms (Ln) was carried out with Voronoi-Dirichlet polyhedra. A novel method for the evaluation of the sizes of polyatomic ligands and steric effects in the structure of organometallic compounds was developed. The dependence of the domain size for Ln atoms on their nature, coordinating number and oxidation state was studied. The reasons for the occurrence of agostic Ln-H contacts were considered with a geometrical-topological analysis.