• bond theory;
  • density functional calculations;
  • energy decomposition analysis;
  • molecular geometry;
  • transition-metal complexes;
  • π backdonation


We have investigated the molecular geometries of a series of dicoordinated d10-transition-metal complexes ML2 (M=Co, Rh, Ir, Ni, Pd, Pt, Cu+, Ag+, Au+; L=NH3, PH3, CO) using relativistic density functional theory (DFT) at ZORA-BLYP/TZ2P. Not all complexes have the expected linear ligand–metal–ligand (L[BOND]M[BOND]L) angle: this angle varies from 180° to 128.6° as a function of the metal as well as the ligands. Our main objective is to present a detailed explanation why ML2 complexes can become bent. To this end, we have analyzed the bonding mechanism in ML2 as a function of the L[BOND]M[BOND]L angle using quantitative Kohn–Sham molecular orbital (MO) theory in combination with an energy decomposition analysis (EDA) scheme. The origin of bent L[BOND]M[BOND]L structures is π backdonation. In situations of strong π backdonation, smaller angles increase the overlap of the ligand’s acceptor orbital with a higher-energy donor orbital on the metal-ligand fragment, and therefore favor π backdonation, resulting in additional stabilization. The angle of the complexes thus depends on the balance between this additional stabilization and increased steric repulsion that occurs as the complexes are bent.