Ambidentate Thiocyanate and Cyanate Ligands in Dinitrosyl Iron Complexes

To explore the effect of delocalization in the Fe­(NO)₂ unit on possible linkage isomerism of ambidentate ECN^– ligands, E = S and O, anionic DNICs, dinitrosyl iron complexes, (SCN)₂Fe­(NO)₂^– (<b>1</b>) and (OCN)₂Fe­(NO)₂^– (<b>2</b>) were synthesized by the reaction of <i>in situ</i>-generated [Fe­(CO)₂(NO)₂]⁺ and PPN⁺ECN^–. Other {Fe­(NO)₂}⁹ (Enemark–Feltham notation) complexes, (N₃)₂Fe­(NO)₂^– and (PhS)₂Fe­(NO)₂^–, were prepared for comparison. The X-ray diffraction analysis of <b>1</b> and <b>2</b> yielded the typical tetrahedral structures of DNICs with two slightly bent Fe–N–O oriented toward each other, and linear FeNCE units. The ν­(NO) IR values shift to lower values for <b>1</b> > <b>2</b> > (N₃)₂Fe­(NO)₂^– > (PhS)₂Fe­(NO)₂^–, reflecting the increasing donor ability of the ancillary ligands and consistent with the redox potentials of the complexes, and the small trends in Mössbauer isomer shifts. Computational studies corroborate that the {Fe­(NO)₂}⁹ motif prefers N-bound rather than E-bound isomers. The calculated energy differences between the linkage isomers of <b>1</b> (Fe-NCS preferred over Fe-SCN by about 6 kcal/mol) are smaller than those of <b>2</b> (Fe-NCO preferred over Fe-OCN by about 16 kcal/mol), a difference that is justified by the frontier molecular orbitals of the ligands themselves