Terminal coordination of diatomic boron monofluoride to iron

Boron monofluoride (BF) is a diatomic molecule with 10 valence electrons, isoelectronic to carbon monoxide (CO). Unlike CO, which is a stable molecule at room temperature and readily serves as both a bridging and terminal ligand to transition metals, BF is unstable below 1800°C in the gas phase, and its coordination chemistry is substantially limited. Here, we report the isolation of the iron complex Fe(BF)(CO)₂(CNArTripp2)₂ [ArTripp2, 2,6-(2,4,6-(i-Pr)₃C₆H₂]₂C₆H₃; i-Pr, iso-propyl], featuring a terminal BF ligand. Single-crystal x-ray diffraction as well as nuclear magnetic resonance, infrared, and Mössbauer spectroscopic studies on Fe(BF)(CO)₂(CNArTripp2)₂ and the isoelectronic dinitrogen (N₂) and CO complexes Fe(N₂)(CO)₂(CNArTripp2)₂ and Fe(CO)₃(CNArTripp2)₂ demonstrate that the terminal BF ligand possesses particularly strong σ-donor and π-acceptor properties. Density functional theory and electron-density topology calculations support this conclusion.