Structure and coordinate bonding nature of the manganese–σ–borane complexes

The electronic and molecular structures of the complexes [(Cp′)Mn(CO)2(σ-HBcat)] (1), [(Cp′)Mn(CO)2(σ-HBpin)] (2) and [(Cp′)Mn(CO)2(σ-HBMe2)] (3) (Cp′ = η5-MeC5H4) have been investigated at the DFT B3LYP and BP86 levels in order to understand the structures, bonding and energetics of the interactions between a transition metal and a σ-HBR2 ligand. The calculated geometries are in excellent agreement with experimental values. These results are consistent with the description of [(Cp′)Mn(CO)2(η2-HBR2)] as a Mn(I) complexes in which both hydrogen and boron of the [HBR2] ligands have a bonding interaction with the manganese and B–H bond character is preserved. Upon coordination of [HBR2], reduction in B–H bond order of about 1/3 was calculated. The LUMO of the distorted HBR2 ligand is not a pure pπ orbital, but is predominantly a hybrid (pπ-s) from boron orbitals with some contribution from H s-orbitals. The coordination of the σ-borane ligand causes a rehybridization of the boron center. NBO analysis shows that the hybrid orbital of B–H bond obtains less 2s character and more 2p character upon coordination of borane to the manganese atom. The predominant effects of the coordination of the σ-borane ligand to manganese are the decrease in the B–H bond order and accompanying increase in the B–H bond distance. The nature of the metal–ligand interactions is quantitatively analyzed with an energy decomposition method. The [(Cp′)Mn(CO)2]-[η2-H-BR2] bonding in borane complexes 1–3 is more than half electrostatic. Indeed the three center-two electron bond in the Mn–H–B bridge may be regarded as a “protonated π-bond”. The metal-η2-H-BR2 moiety in borane complexes is, therefore, analogous to metal boryl complexes with M–B σ and π bonds with a boron atom lying in the plane defined by the metal and two R substituents.