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The mixed isocyanide/carbonyl complexes cis - and trans -[Re(CO) 3 Br(CNAr Dipp2 ) 2 ] (Ar Dipp2 = 2,6-(2,6-(i-Pr) 2 C 6 H 3 ) 2 C 6 H 3 ) can be synthesized from reactions of [Re(CO) 5 Br] and CNAr Dipp2 depending on the conditions applied. Reduction of the neutral Re( i ) species gives the monoanionic complex [Re(CO) 3 (CNAr Dipp2 ) 2 ] − or the neutral [Re(CO) 3 (CNAr Dipp2 ) 2 ], which contain rhenium in the formal oxidation states “−1” and “0”, respectively. 

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 (CNAr Tripp2 ) 2 [Ar Tripp2 , 2,6-(2,4,6-( i- Pr) 3 C 6 H 2 ] 2 C 6 H 3 ; 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 (CNAr Tripp2 ) 2 and the isoelectronic dinitrogen (N 2 ) and CO complexes Fe(N 2 )(CO) 2 (CNAr Tripp2 ) 2 and Fe(CO) 3 (CNAr Tripp2 ) 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.