Group 11 metal complexes of N-heterocyclic carbene ligands: Nature of the metal-carbene bond

The silver complex of the tripodal N-heterocyclic carbene ligand TIMEMe, [(TIMEMe)2Ag3](PF6)3 (3), reacts with copper(I) bromide and (dimethyl sulfide)gold(I) chloride to yield the corresponding D3-symmetrical copper(I) and gold(I) complexes [(TIMEMe)2Cu3](PF6)3 (4) and [(TIMEMe)2Au3](PF6)3 (5). Single-crystal X-ray diffraction, spectroscopic, and computational studies of this series of metal NHC complexes are described. The group 11 metal complexes of the TIMEMe ligand exhibit isostructural geometries, with three metal ions bridging two of the TIMEMe ligands. Each metal ion is linearly coordinated to two carbene centers, with each of the carbenoid carbons stemming from a different ligand. Overall, the molecules possess D3 symmetry. The electronic structure of these newly synthesized compounds was elucidated with the aid of DFT calculations. In contrast to the common assumption that NHCs are pure σ-donor ligands, our calculations reveal the existence of both σ- and π-type interactions between the metal ions and the carbenoid carbons. A study of the closely related D2d-symmetrical species Pd(CN2But2C2H2) 2 (6) and the simplified D2h-symmetrical model complexes M(IMMeC:)2 (8-10; M = Ag, Cu, Au) allowed for quantitative comparison of the two different types of bonding interactions. It was found that π-back-bonding interactions in these diaminocarbene model species contribute to approximately 15-30% of the complexes' overall orbital interaction energies.