Bonding Analysis of TM(cAAC)₂ (TM = Cu, Ag, and Au) and the Importance of Reference State

A recent analysis of the bonding in transition metal (TM) complexes with cyclic aminoalkyl carbene (cAAC) ligands, TM­(cAAC)2 (TM = Cu, Ag, and Au), purports to show that metal–ligand bonding involves the TM in the excited 2P state and that TM­(pπ) → (cAAC)2 backdonation is not properly recognized in NBO analysis because of biases against participation of np functions in transition metal bonding. The questions of TM np orbital involvement in bonding and the possible biases in the NBO occupancy-weighted symmetric orthogonalization procedure have been examined by performing NBO analyses in two ways: (1) single Lewis structure (loc) analysis with TM np orbitals treated as valence (NBOs) or nonvalence (NBOx) and (2) direct comparison of a two-configuration resonance model (res/NBOs) treatment with a single configuration model using the expanded valency (loc/NBOx) treatment. The principal bonding picture that emerges from NBO analysis features a TM cation with two “non-innocent” cAAC ligands that are each reduced by 0.5 electrons. The unpaired spin delocalizes over a π network spanning the two ligands, whether or not a TM cation is present. In the localized NBO framework, the unpaired spin primarily occupies a 1e π-type “long-bond” between the carbonic carbon centers, with secondary resonance delocalization over the TM npπ and the two Npπ orbitals. This description is consistent with all experimental data. Energy decomposition analysis–natural orbitals for chemical valence (EDA-NOCV) analysis of the Cu complex with different reference states reveals that the inferred nature of the bonding depends wholly on the choice of reference state. We show that the earlier selection of a neutral, excited 2P Cu reference state virtually dictates the bonding description to feature an unphysical degree of TM­(pπ) → (cAAC)2 backdonation.