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)Pd 0 (C 2 H 4 )] and [(L)Pd II (C 2 H 4 )] 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 C C bond activation are strongly correlated with increasing ligand basicity, including the C C bond length and vibrational frequency, the magnitude of Pd(0) → ethylene( π ∗ ) NBO donation, and the Wiberg C C 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 C C bond length and ν (CC). EDA-NOCV results show that the [(L)Pd 0 ]-(C 2 H 4 ) 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)Pd 0 ] HOMO with ligand p K a is directly correlated with the enhanced [(L)Pd 0 ] → ethylene( π ∗ ) donation. For the [(L)Pd II (C 2 H 4 )] 2+ adducts, C C bond lengthening and weakening is roughly half the amount observed with Pd 0 , with ligand substitution having a negligible impact. NBO analysis confirms that ethylene activation is dominated by ethylene( π ) → Pd II donation, which is enhanced with electron-withdrawing ligands while Pd II  → ethylene( π ∗ ) donation decreases with less basic ligands. EDA-NOCV results show that the [(L)Pd II ] 2+ -(C 2 H 4 ) 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)Pd II ] 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)Pd II ] 2+ fragment.