Terminal Hydride Complex of High-Spin Mn

The iron-molybdenum cofactor of nitrogenase (FeMoco) catalyzes fixation of N₂ via Fe hydride intermediates. Our understanding of these species has relied heavily on the characterization of well-defined 3d metal hydride complexes, which serve as putative spectroscopic models. Although the Fe ions in FeMoco, a weak-field cluster, are expected to adopt locally high-spin Fe^2+/3+ configurations, synthetically accessible hydride complexes featuring d⁵ or d⁶ electron counts are almost exclusively low-spin. We report herein the isolation of a terminal hydride complex of four-coordinate, high-spin (d⁵; S = 5/2) Mn²⁺. Electron paramagnetic resonance and electron-nuclear double resonance studies reveal an unusually large degree of spin density on the hydrido ligand. In light of the isoelectronic relationship between Mn²⁺ and Fe³⁺, our results are expected to inform our understanding of the valence electronic structures of reactive hydride intermediates derived from FeMoco.