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

Abstract

The electronic and molecular structures of the complexes [(MeC 5H 4)Re(CO) 2(σ-HBcat)] 1, [(MeC 5H 4)Re(CO) 2(σ-HBpin)] 2 and [(MeC 5H 4)Re(CO) 2(σ-HBMe 2)] 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-HBR 2 ligand. The nature of the Re–η 2-HBR 2 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 [(η 5-MeC 5H 4)Re(CO) 2(η 2-HBR 2)]: 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 [(MeC 5H 4)Re(CO) 2(η 2-HBR 2)] as a Re(I) complexes in which both hydrogen and boron of the [HBR 2] ligands have a bonding interaction with the rhenium and B–H bond character is preserved. Upon coordination of [HBR 2], 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 [(η 5-MeC 5H 4)Re(CO) 2(HBcat)], −39.7 kcal/mol for [(η 5-MeC 5H 4)Re(CO) 2(HBpin)] and −43.3 kcal/mol for [(η 5-MeC 5H 4)Re(CO) 2(HBMe 2)]. The [(η 5-MeC 5H 4)Re(CO) 2]–[η 2-H-BR 2] 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–η 2-H-BR 2 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.