Data on molecular design of various types of mononuclear complexes of Schiff bases are generalized. Factors for variation of fine structure of Schiff-base ligand systems (metallocycle sizes and their number, nature of donor centers, aldehyde and amine fragments, co-ligands) are examined. A systematization of Salen complexes is offered. Electronic configurations of p- and d-metals in the coordination and organometallic compounds are taken into account in a series of chelate structures. Non-standard structures are emphasized, including various ligand chelates and molecular complexes.