[en] Experimental and theoretical data on the structures and properties of compounds of tetravalent transactinides TAn^IV: Rf (Z=104), Db (Z=105), Sg (Z=106), Bh (Z=107), Hs (Z=108), Mt (Z=109), and ¹¹⁰TAn (Z=110) are described systematically. The problem of description of TAn in the Periodic System of Elements is discussed and the properties of 6d-, 5d-, 4d-, 3d-, and 5f-elements are compared. The regular features in the variation of thermodynamic characteristics of TAn^IV compounds vs. the atomic number Z(TAn) are elucidated. The changes in the thermodynamic properties upon the formation of nitrate, fluoride and chloride TAn⁴ complexes in solution are analyzed