Stretched σ-borane complexes of rhodium: A theoretical study

A theoretical study of the stretched σ-borane complexes [(η5-C5H5)Rh(HBpin)2] 1Rh and [(η5-C5H5)Rh(HBpin)(Bpin)2] 2Rh (HBpin, pinacolborane) at the DFT BP86 levels has been carried out. In contrast to the previously optimized geometry of [(η5-C5H5)Rh(HBO2C2H4)2], the optimized structure of 1Rh contains almost similar HBpin ligands. The B–H bond distances B1–H1, 1.696 Å and 1.747 Å in 1Rh and 1.686 Å in 2Rh are longer than expected for a single bond based on covalent radius predictions (1.19 Å) The B1–Rh–H1 bond angles 55.6° and 52.6° in 1Rh and 2Rh, respectively and B2–Rh–H2 bond angle in 1Rh are larger than the B–M–H bond angles in σ-borane complexes (37–41°) and are consistent with rhodium–σ-borane bonding, preserving some B–H bond character. The Rh–B and Rh–H bonding orbitals are strongly polarized towards rhodium.