Monohaloboryls (BHX−) as Bridging Ligands: Observable Dinuclear σ‐(Halo)boranyl Intermediates in the Synthesis of Metalloborylenes

Upon treating transition‐metal–dihaloboryl complexes of the form [L_nMBX₂] with K[(η⁵‐C₅H₅)MnH(CO)₂], salt elimination occurs along with a migration of the Mn‐bound hydride ligand onto the boron atom, thereby forming dinuclear σ‐(halo)boranyl complexes of the form [L_nM(μ‐BHX)Mn(CO)₂(η⁵‐C₅H₅)]. Most of these complexes react further at room temperature to lose HX and provide metalloborylene complexes [L_nM‐B=Mn(CO)₂(η⁵‐C₅H₅)]; however, when ML_n=Re(CO)₅ the σ‐(halo)boranyl complex decomposes into unidentifiable products. We found through DFT calculations that two electronically and structurally distinct forms of the intermediate σ‐(halo)boranyl complexes exist, one of which easily loses HX and one that does not.