Synthesis and Molecular Structures of Mononitrosyl (N₂S₂)M(NO) Complexes (M = Fe, Co)

A series of tetragonally distorted square pyramids of formula N2S2M(NO) (M = Fe, Co) is prepared and characterized by ν(NO) IR and EPR spectroscopies, magnetism and electrochemical properties, as well as solid-state crystal structure determinations. While the ν(NO) IR frequencies and the ∠M−N−O angles indicate differences in the electronic environment of NO consistent with the Enemark−Feltham notation of {Fe(NO)}7 and {Co(NO)}8, the reduction potentials, assigned to {Fe(NO)}7 + e- ⇌ {Fe(NO)}8 and {Co(NO)}8 + e- ⇌ {Co(NO)}9 respectively, are very similar, and in cases identical, for most members of the series. Coupled with the potential for the M(NO) units to breathe out of and into the N2S2 core plane are unique S−M−N−O torsional arrangements and concomitant π-bonding interactions which may account for the unusual coherence of reduction potentials within the series.