Group 11 Metal Complexes of N-Heterocyclic Carbene Ligands: Nature of the MetalCarbene Bond
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posted on 2004-02-16, 00:00 authored by Xile Hu, Ingrid Castro-Rodriguez, Kristian Olsen, Karsten MeyerThe 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.
