N,N-coordinated pi radical anions of S-methyl-1-phenyl-isothiosemicarbazide in two five-coordinate ferric complexes [Fe III(L Me*)(2)X] (X = CH3S-, Cl-).

The reaction between Fe(III)(dmf)(6)(3) and the ligand S-methyl-1-phenyl-isothiosemicarbazide, H(2)[L(Me)], and triethylamine (1:3:6) in methanol under an argon blanketing atmosphere at elevated temperatures (reflux) yields a purple solution from which upon cooling to 20 degrees C dark green crystals of [Fe(III)(L(Me)())(2)(SCH(3))] (1) were obtained in 15% yield. From a similar reaction mixture using FeCl(3) as starting material in the solvent acetone under anaerobic conditions at -80 degrees C, dark green crystals of [Fe(III)(L(Me)())(2)Cl] (2) were obtained in 21% yield. The structures of complexes 1 and 2 have been determined by single-crystal X-ray crystallography at 100 K. Both complexes are five-coordinate square base pyramidal ferric species containing two N,N-coordinated, monoanionic pi radicals, (L(Me)(*))(1)(-), of the parent S-methyl-1-phenyl-isothiosemicarbazide(2-) dianion in the basal positions whereas the axial position is occupied by methylthiolate in 1 and chloride in 2, respectively. The electronic structure of both species has been elucidated by their electronic spectra, magnetic properties, and X-band EPR and Mössbauer spectra. Both possess an S(t) = (1)/(2) ground state which is attained via an antiferromagnetic coupling between the spins of an intermediate spin ferric ion (S(Fe) = (3)/(2)) and two ligand pi radical anions (S(rad) = (1)/(2)).