Tuning the oxidation state and magnetic and coordination behaviour of iron and cobalt complexes by O/S variation in mono-thio and dithio-oxamide chelating ligands

New iron and cobalt complexes coordinated with the ligands Me(2)pipto (N,N'-dimethyl-piperazine-3-oxo-2-thione) and Me(2)pipdt (N,N'-dimethyl-piperazine-2,3-dithione,S,S'), differing in one sulphur substituting the oxygen atom, have been prepared and characterized. The reaction with Me(2)pipto and iron salts affords the heteroleptic [Fe-III(Me(2)pipto)(2)Cl-2](+) or the homoleptic [FeII(Me(2)pipto)(3)](2+) cationic complexes (isolated as tetrafluoroborate salts 1 and 2) depending on the employed iron source: FeCl3 or Fe-2(SO4)(3) respectively. The corresponding reaction with CoCl2 as the metal source allowed us to obtain [Co-II(Me(2)pipto)(3)](BF4)(2) (3). By reacting FeCl3, Me(2)pipto and KSCN in the molar ratio 1:1:4, [FeIII(Me(2)pipto)(NCS)(4)](-) is obtained and isolated as a Ph4P+ salt (4). Upon using the Me(2)pipdt ligand, the reaction with FeC(l)3 affords [FeII(Me(2)pipdt)(3)](2+) which is isolated as a tetrafluoroborate salt (5). The same cation is found in the polyiodide salt [Fe-II(Me(2)pipdt)(3)](I-3)(1.8)(I)(0.2) (6) obtained by reacting iron-metal powders with Me(2)pipdt and I-2 mixtures. Through a similar reaction by using cobalt-metal powders, [CoIII(Me(2)pipdt)(3)](2)(I-3)(2)(I)(4)center dot 2I(2) (7) is obtained. Structural results show that in all these compounds the metal ions are in a pseudo-octahedral coordination geometry and that bond distances are consistent with the presence of iron(III) in 1 and 4 and iron(II) in 2 and cobalt(III) in 7. Magnetic susceptibility measurements show that the metals are in a high spin state in all Me2pipto complexes and a low spin state in Me(2)pipdt. The observed differences are relatable to the different sigma-donor and pi-acceptor capabilities of the ligands tuned by the S and O donor atoms. Results from DFT calculations using B3LYP and OLYP as functionals are in agreement with the observed magnetic behaviour of the complexes.