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 (P 2 ) should more closely mimic the attributes of acetylene (HC≡CH) rather than its group 15 congener dinitrogen (N 2 ). Although acetylene has well-documented coordination chemistry with mononuclear transition metals, coordination complexes that feature P 2 bound to a single metal center have remained elusive. We report the isolation and x-ray crystallographic characterization of a mononuclear iron complex featuring P 2 coordination in a side-on, η 2 -binding mode. An analogous η 2 -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 η 2 -P 2 and η 2 -acetylene ligands exert a similar electronic demand on mononuclear iron centers but exhibit different reactivity profiles.