A systematic examination of ligand basicity effects on bonding in palladium(0)- and palladium(II)-ethylene complexes

Abstract

The impact of ancillary ligand basicity on the binding and activation of ethylene in a series of [(L)Pd0(C2H4)] and [(L)PdII(C2H4)]2+ complexes (where L = substituted 1,10-phenanthroline ligands) has been studied with density functional theory using natural bond orbital (NBO) analysis, energy decomposition analysis (EDA-NOCV), and molecular orbital (MO) analysis. With palladium(0), metrics of ethylene CC bond activation are strongly correlated with increasing ligand basicity, including the CC bond length and vibrational frequency, the magnitude of Pd(0) → ethylene(π∗) NBO donation, and the Wiberg CC bond index. In contrast, ethylene(π) → Pd(0) donation is not strongly influenced by ligand basicity, which is manifested in strong correlations between Pd(0) → ethylene(π∗) donation and both CC bond length and ν(CC). EDA-NOCV results show that the [(L)Pd0]-(C2H4) interaction energy is dominated by Coulombic attraction and enhanced slightly with more electron-donating ligands. MO analysis demonstrates that a linear increase in the energy of the filled [(L)Pd0] HOMO with ligand pKa is directly correlated with the enhanced [(L)Pd0] → ethylene(π∗) donation. For the [(L)PdII(C2H4)]2+ adducts, CC bond lengthening and weakening is roughly half the amount observed with Pd0, with ligand substitution having a negligible impact. NBO analysis confirms that ethylene activation is dominated by ethylene(π) → PdII donation, which is enhanced with electron-withdrawing ligands while PdII → ethylene(π∗) donation decreases with less basic ligands. EDA-NOCV results show that the [(L)PdII]2+-(C2H4) interaction energy is also dominated by Coulombic attraction and is enhanced by more electron-withdrawing ligands, largely as a result of a concurrent increase in ethylene(π) → [(L)PdII]2+ stabilization and a decrease in Pauli repulsion. Finally, MO analysis indicates an electrophilic ethylene moiety due to substantial stabilization of both the alkene π- and π∗-orbitals through interaction with the [(L)PdII]2+ fragment.