σ-Borane complexes of nickel, palladium and platinum: A theoretical study

Quantum chemical calculations at the gradient corrected DFT level using the exchange correlation functionals BP86 of the complexes [(dmpe)M(HBR2)] (M = Ni, Pd, Pt; R = Et, Me) are reported. The calculated electronic and molecular structures of the complexes [(dmpe)M(HBR2)] (M = Ni, Pd; R = Et, Me) are consistent with [(dmpe)M(η2-HBR2)] being Ni(0) and Pd(0) complexes in which both hydrogen and boron of the [HBR2] ligands have a bonding interaction with the metal preserving B-H bond character. The results of the theoretical investigation suggest that the complex [(dmpe)Pt(HBEt2)] is a platinum(II) hydride boryl complex rather than σ-borane complex, while complex [(dmpe)Pt(HBMe2)] with some residual B-H interaction, is an example of elongated σ-borane complex. The nature of the metal-ligand interactions is quantitatively analyzed with an energy decomposition analysis. The bond dissociation energy is slightly larger in [(dmpe)M(η2-HBMe2)] than in [(dmpe)M(η2-HBEt2)] (M = Ni, Pd). The values of interaction energy, ΔEint as well as orbital interactions ΔEorb decrease on going from nickel to palladium. For M-η2-H-BR2 (M = Ni, Pd) bonds, the contribution of electrostatic attractions ΔEelstat are greater than the orbital interactions, ΔEorb. The repulsive terms ΔEPauli were larger in each case. All four [(dmpe)M(HBR2)] (M = Ni, Pd; R = Et, Me) complexes exhibit about 40-44% covalent bonding of the borane ligand to the metal fragment. For the platinum complexes [(dmpe)Pt(HBR2)] (R = Et, Me), the preparation energy, ΔEprep as well as interaction energy, ΔEint and its components, ΔEPauli, ΔEelstat, and ΔEorb are large, since the HBR2 unit near the dissociation limit. The complex [(dmpe)Pt(HBMe2)] is intermediate between σ-borane complexes and hydride boryl complex.