QTAIM analysis of ligand properties and mechanisms of tuning of 6-membered ring N-heterocyclic carbenes in transition metal complexes through ring-substituent variation.
Tuning of the ligand properties of a series of N-heterocyclic carbenes has previously been achieved through the variation of substituents at the ring boron sites. Analysis of the topology of the electron density using the quantum theory of atoms in molecules (QTAIM) reveals sensitive changes in the integrated atomic properties and the curvature of the valence shell charge concentration of the carbene carbon (C2). Amino substitution induces greater ligand-to-metal sigma-donation in the transition metal complex, which in turn weakens the attractive potential energy density on the interatomic surface of the trans-carbonyl group leading to the decrease in the trans C-O stretching frequency observed experimentally and theoretically. The distribution of charge concentrations within the inner-valence shell of cobalt in the transition metal complexes recovers the QTAIM analogs of ligand-to-metal sigma-donation and metal-to-ligand pi-back-donation. Investigation of the virial of the Ehrenfest force acting on the interatomic surfaces shows the operative mechanism of ligand tuning to be the inductive withdrawal of charge from the ring by nitrogen and subsequent back-polarization of remaining charge toward C2. This mechanism is at odds with the orbital viewpoint of exo nitrogen-to-ring-boron pi-back-donation; however, it is the electronic forces that govern the bonding and charge distribution within a molecule in a stationary state.
Johnson
Lucius E
LE
Department of Chemistry, University of Louisville, Louisville, Kentucky 40292, USA.
DuPré
Donald B
DB
D006571:Heterocyclic Compounds / Q000737:chemistry*; D008024:Ligands; D008697:Methane / Q000031:analogs & derivatives* / Q000737:chemistry; D009942:Organometallic Compounds / Q000737:chemistry*; D011789:Quantum Theory; D028561:Transition Elements / Q000737:chemistry*
D006571:Heterocyclic Compounds; D008024:Ligands; D009942:Organometallic Compounds; D028561:Transition Elements; C030011:carbene; D008697:Methane