What is the best bonding model of the (σ-H-BR) species bound to a transition metal? Bonding analysis in complexes [(H)2Cl(PMe3)2M(σ-H-BR)] (M = Fe, Ru, Os)

Abstract

Density Functional Theory calculations have been performed for the σ-hydroboryl complexes of iron, ruthenium and osmium [(H)₂Cl(PMe₃)₂M(σ-H-BR)] (M = Fe, Ru, Os; R = OMe, NMe₂, Ph) at the BP86/TZ2P/ZORA level of theory in order to understand the interactions between metal and HBR ligands. The calculated geometries of the complexes [(H)₂Cl(PMe₃)₂Ru(HBNMe₂)], [(H)₂Cl(PMe₃)₂Os(HBR)] (R = OMe, NMe₂) are in excellent agreement with structurally characterized complexes [(H)₂Cl(PⁱPr₃)₂Os(σ-H-BNMe₂)], [(H)₂Cl(PⁱPr₃)₂Os{σ-H-BOCH₂CH₂OB(O₂CH₂CH₂)}] and [(H)₂Cl(PⁱPr₃)₂Os(σ-H-BNMe₂)]. The longer calculated M–B bond distance in complex [(H)₂Cl(PMe₃)₂M(σ-H-BNMe₂)] are due to greater B–N π bonding and as a result, a weaker M–B π-back-bonding. The B–H2 bond distances reveal that (i) iron complexes contain bis(σ-borane) ligand, (ii) ruthenium complexes contain (σ-H-BR) ligands with a stretched B–H2 bond, and (iii) osmium complexes contain hydride (H2) and (σ-H-BR) ligands. The H-BR ligands in osmium complexes are a better trans-directing ligand than the Cl ligand. Values of interaction energy, electrostatic interaction, orbital interaction, and bond dissociation energy for interactions between ionic fragments are very large and may not be consistent with M–(σ-H-BR) bonding. The EDA as well as NBO and AIM analysis suggest that the best bonding model for the M–σ-H-BR interactions in the complexes [(H)₂Cl(PMe₃)₂M(σ-H-BR)] is the interaction between neutral fragments [(H)₂Cl(PMe₃)₂M] and [σ-H-BR]. This becomes evident from the calculated values for the orbital interactions. The electron configuration of the fragments which is shown for C in Fig. 1 experiences the smallest change upon the M–σ-H-BR bond formation. Since model C also requires the least amount of electronic excitation and geometry changes of all models given by the ΔE_prep values, it is clearly the most appropriate choice of interacting fragments. The π-bonding contribution is 14–22% of the total orbital contribution.