Side-on coordination of diphosphorus to a mononuclear iron center

The diagonal relationship in the periodic table between phosphorus and carbon has set an expectation that the triple-bonded diatomic diphosphorus molecule (P₂) should more closely mimic the attributes of acetylene (HC≡CH) rather than its group 15 congener dinitrogen (N₂). Although acetylene has well-documented coordination chemistry with mononuclear transition metals, coordination complexes that feature P₂ bound to a single metal center have remained elusive. We report the isolation and x-ray crystallographic characterization of a mononuclear iron complex featuring P₂ coordination in a side-on, η²-binding mode. An analogous η²-bound bis-timethylsilylacetylene iron complex is reported for comparison. Nuclear magnetic resonance, infrared, and Mössbauer spectroscopic analysis-in conjunction with density functional theory calculations-demonstrate that η²-P₂ and η²-acetylene ligands exert a similar electronic demand on mononuclear iron centers but exhibit different reactivity profiles.