Fe=B double bonds: Synthetic, structural, and reaction chemistry of cationic terminal borylene complexes

Application of halide abstraction chemistry to asymmetric haloboryl complexes (eta(5)-C5Me5)Fe(CO)(2)B(ERn)X leads to the first synthetic route to cationic multiply bonded group 13 diyl species, [(eta(5) -C5Me5)Fe(CO)(2)B(ERn)](+). The roles of steric bulk and pi electron release within the ERn substituent in generating tractable borylene complexes have been probed, as has the nature of the counterion. A combination of spectroscopic, structural, and computational techniques leads to the conclusion that the bonding in complexes such as [(eta(5)-C5Me5)Fe(CO)(2)B(Mes)](+) is best described as an Fe=B double bond composed of B-->Fe sigma donor and Fe-->B pi back-bonding components. An extended study of the fundamental reactivity of cationic borylene systems reveals that this is dominated not only by nucleophilic addition at boron but also by iron-centered substitution chemistry leading to overall displacement of the borylene ligand.