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

The electronic and molecular structures of the complexes [(MeC5H4)Re(CO)2(σ-HBcat)] 1, [(MeC5H4)Re(CO)2(σ-HBpin)] 2 and [(MeC5H4)Re(CO)2(σ-HBMe2)] 3 have been investigated at the DFT BP86/TZ2P levels in order to understand the structures, bonding and energetic of the interactions between a transition metal and a σ-HBR2 ligand. The nature of the Re-η2-HBR2 interactions was analyzed with charge and energy decomposition methods. The Re-B, B-H, Re-H bond distances are longer than that expected for single bond based on covalent radius predictions. The B-Re-H1 bond angles in the complexes [(η5-MeC5H4)Re(CO)2(η2-HBR2)]: 40.8° in 1, 41.5° in 2 and 38.5° in 3 are small and are consistent with B-H bonding. These results are consistent with the description of [(MeC5H4)Re(CO)2(η2-HBR2)] as a Re(I) complexes in which both hydrogen and boron of the [HBR2] ligands have a bonding interaction with the rhenium and B-H bond character is preserved. Upon coordination of [HBR2], reduction in B-H bond order about 1/2 was calculated. The theoretically predicted bond dissociation energies of the borane ligands are −40.5 kcal/mol for [(η5-MeC5H4)Re(CO)2(HBcat)], −39.7 kcal/mol for [(η5-MeC5H4)Re(CO)2(HBpin)] and −43.3 kcal/mol for [(η5-MeC5H4)Re(CO)2(HBMe2)]. The [(η5-MeC5H4)Re(CO)2]-[η2-H-BR2] bonding in borane complexes 1-3 is more than half electrostatic. All three complexes exhibit about 42-44% covalent bonding of the borane ligand to the metal fragment. Indeed the three center-two electron bond in the Re-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.