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New magnesium complexes [Mg(H2O)2(ofhac)2] 1 and [Mg(tmeda)(ofhac)2] 2 (ofhac = , tmeda = N,N,N′,N′-tetramethylethylenediamine) with a 1,1,1,2,2,6,6,6-octafluorohexane-3,5-dionate ligand are prepared. The composition of these compounds is confirmed by elemental analysis and IR spectroscopy, the structure is determined by XRD. The coordination environment of magnesium is distorted octahedral in both complexes. The ofhac ligands are coordinated in the bidentate-cyclic mode; the lengths of Mg–O bonds are similar and vary within 2.033(14)-2.063(18) Å. The aqua ligands in 1 occupy cis positions (d(Mg–O) = 2.0511(18) Å, θ(O–Mg–O) = 85.0(1)°) and participate in the system of O–H...O and O–H...F hydrogen bonds forming a chain packing. The tmeda ligand in 2 performs a chelating function (d(Mg–N) = 2.212(4) Å, θ(N–Mg–N) = 81.0(6)°). The dependence of the structure and the thermal properties of the complexes on the size of the fluorinated substituent (C2F5 instead of CF3) is estimated by comparing the complex with its analogues bearing 1,1,1,5,5,5-hexafluoro-2,4-pentadionate ligands. The crystal structures of [Mg(H2O)2(L)2] are homeotypic; the analogues of [Mg(tmeda)(L)2] crystallize in different space groups. The absence of trans isomers in [Mg(H2O)2(L)2] is confirmed by powder XRD. The thermogravimetry experiments in flowing helium and sublimation tests in vacuum showed that complexes with the ofhac ligand are more volatile. Introducing a C2F5 groups into the ligand also decreases the melting point of the compounds, more significantly for aqua complexes. [ABSTRACT FROM AUTHOR]