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

Abstract

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 2 P 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 (NBO s ) or nonvalence (NBO x ) and (2) direct comparison of a two-configuration resonance model (res/NBO s ) treatment with a single configuration model using the expanded valency (loc/NBO x ) 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 2 P Cu reference state virtually dictates the bonding description to feature an unphysical degree of TM(pπ) → (cAAC) 2 backdonation. ■ INTRODUCTIONIn recent publications, Jerabek, Roesky, Bertrand, Frenking (JRBF) and coauthors 1−3 demonstrate that cyclic aminoalkylcarbenes (cAACs) are capable of stabilizing coinage metals in the formal oxidation state zero, leading to crystallographically characterized structures for Cu and Au. The neutral coinage transition metals complexes [TM(cAAC) 2 ] (TM = Cu, Ag, and Au), which are generated by reduction of the [TM(cAAC) 2 ] + cations for TM = Cu,Au, exhibit interesting experimental and computational metrics, linear coordination geometries, EPR gvalues, and hyperfine couplings, indicating that the unpaired spin density is largely located on the carbene carbon atoms and that C-TM bond lengths that are slightly longer for the [TM(cAAC) 2 ] + cations than for the neutral complexes. A recent analysis of the bonding in these complexes using energy decomposition analysis−natural orbitals for chemical valence (EDA-NOCV) 4−8 and natural bond orbital (NBO) methods 9−15 prompted several claims from the authors that appear contrary to the empirical data and to previously published examinations of bonding at transition metals. Representative statements include "Metal-ligand interactions in [TM...

Comparison of Hydrogen and Gold Bonding in [XHX]−, [XAuX]−, and Isoelectronic [NgHNg]+, [NgAuNg]+(X=Halogen, Ng=Noble Gas)

“…[51] The conclusion was challenged on the basis of an NBO analysis, saying that the EDA-NOCV study wronglyu sed the excited 2 Ps tate of Au as ar eference state. [52] There is no doubt about the electronic reference state of Au in the present case, because the neutral Au or charged Au + + is considered to be in the ground state.…”

What is NBO analysis and how is it useful?

“…In contrast, the simple Lewis-like and resonance conceptions that emerge from NBO/NRT analysis of TM(cAAC) 2 bonding [53] are fully consistent with known experimental properties of these species, as well as with broader valency and bonding principles that apply across the periodic table [15]. In contrast, the simple Lewis-like and resonance conceptions that emerge from NBO/NRT analysis of TM(cAAC) 2 bonding [53] are fully consistent with known experimental properties of these species, as well as with broader valency and bonding principles that apply across the periodic table [15].…”

“…In this study, the authors employ Ziegler-type EDA-NOCV ('natural orbitals for chemical valency' [51]) to draw a number of remarkable conclusions, including statements such as: Similar statements concerning NBO methods were also echoed by Frenking and Caramori [52] (viz., 'the NBO method is useless for elucidating the nature of chemical bonding because it affords only information about orbital interactions in a biased way … '). Such statements motivated our search for more direct head-to-head comparisons of NBO vs. EDA-NOCV analyses of TM(cAAC) 2 species that could clarify the origin of the sharp conceptual disagreements [53].…”

Theoretical study of bis(N-(5-(4-methoxyphenyl)-1,3,4-oxadiazol-2-yl)ethanimidamidato)M complexes (M = Co, Ni, Cu, Zn, Pd, Cd): Structural, electronic and optical properties

“…Let us note that in NPA analysis of transition metals, the addition of a set of p-type empty orbitals (referred to as Rydberg orbitals) to the valence orbitals mays lead in a different electronic configuration. The appropriateness of using NPA on metal transition are discussed in more details in Ref [36][37][38]. Here we have used the standard natural atomic orbitals as available in Gaussian09 [31].…”