Coinage metals binding as main group elements: Structure and bonding of the carbene complexes [TM(cAAC)₂] and [TM(cAAC)₂]⁺ (TM = Cu, Ag, Au)

Quantum chemical calculations using density functional theory have been carried out for the cyclic (alkyl)(amino)carbene (cAAC) complexes of the group 11 atoms [TM(cAAC)₂] (TM = Cu, Ag, Au) and their cations [TM(cAAC)₂]⁺. The nature of the metal-ligand bonding was investigated with the charge and energy decomposition analysis EDA-NOCV. The calculations show that the TM-C bonds in the charged adducts [TM(cAAC)₂]⁺ are significantly longer than in the neutral complexes [TM(cAAC)₂], but the cations have much higher bond dissociation energies than the neutral molecules. The intrinsic interaction energies δE_int in [TM(cAAC)₂]⁺ take place between TM⁺ in the ¹S electronic ground state and (cAAC)₂. In contrast, the metal-ligand interactions in [TM(cAAC)₂] involve the TM atoms in the excited ¹P state yielding strong TM p(π) → (cAAC)₂ π backdonation, which is absent in the cations. The calculations suggest that the cAAC ligands in [TM(cAAC)₂] are stronger π acceptors than σ donors. The trends of the intrinsic interaction energies and the bond dissociation energies of the metal-ligand bonds in [TM(cAAC)₂] and [TM(cAAC)₂]⁺ give the order Au > Cu > Ag. Calculations at the nonrelativistic level give weaker TM-C bonds, particularly for the gold complexes. The trend for the bond strength in the neutral and charged adducts without relativistic effects becomes Cu > Ag > Au. The EDA-NOCV calculations suggest that the weaker bonds at the nonrelativistic level are mainly due to stronger Pauli repulsion and weaker orbital interactions. The NBO picture of the C-TM-C bonding situation does not correctly represent the nature of the metal-ligand interactions in [TM(cAAC)₂].