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

The electronic and molecular structures of the complexes [(Cp')Mn(CO)(2)(sigma-HBcat)] (1), [(Cp')Mn(CO)(2)(sigma-HBpin)] (2) and [(Cp')Mn(CO)(2)(sigma-HBMe2)](3) (Cp'= eta(5)-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 sigma-HBR2 ligand. The calculated geometries are in excellent agreement with experimental values. These results are consistent with the description of [(Cp')Mn(CO)(2)(eta(2)-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 p(pi) orbital, but is predominantly a hybrid (p(pi)-s) from boron orbitals with some contribution from H s-orbitals. The coordination of the sigma-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 sigma-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)]-[eta(2)-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 pi-bond". The metal-eta(2)-H-BR2 moiety in borane complexes is, therefore, analogous to metal boryl complexes with M-B sigma and pi bonds with a boron atom lying in the plane defined by the metal and two R substituents. (c) 2007 Elsevier B.V. All rights reserved.