title: Copper and Silver Carbene Complexes without Heteroatom-Stabilization: Structure, Spectroscopy, and Relativistic Effects.
abstract: Salts of a copper and a silver carbene complex were prepared from dimesityl diazomethane, made possible by the steric shielding of the N-heterocyclic carbene (NHC) ancillary ligand IPr**. The mint-green complex [IPr**Ag=CMes2 ](+) [NTf2 ](-) is the first isolated silver carbene complex without heteroatom donor substituents. Single-crystal X-ray diffraction provides evidence for a predominant carbenoid character, and supports the postulation of such reactive species as intermediates in silver-catalyzed C-H activation reactions. The greenish yellow copper carbene complex [IPr**Cu=CMes2 ](+) [NTf2 ](-) has spectroscopic properties in between the isostructural silver complex and the already reported emerald green gold carbene complex. A comparison in the Group 11 series indicates that relativistic effects are responsible for the strong σ bond and the significant π back-bonding in the gold carbene moiety.
authors.affiliation: Organisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg (Germany) http://www.uni-heidelberg.de/fakultaeten/chemgeo/oci/akstraub.
authors.forename: Matthias W
authors.initials: MW
authors.lastname: Hussong
authors.affiliation: Organisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg (Germany) http://www.uni-heidelberg.de/fakultaeten/chemgeo/oci/akstraub.
authors.forename: Wilhelm T
authors.initials: WT
authors.lastname: Hoffmeister
authors.affiliation: Organisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg (Germany) http://www.uni-heidelberg.de/fakultaeten/chemgeo/oci/akstraub.
authors.forename: Frank
authors.initials: F
authors.lastname: Rominger
authors.affiliation: Organisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg (Germany) http://www.uni-heidelberg.de/fakultaeten/chemgeo/oci/akstraub. Straub@oci.uni-heidelberg.de.
authors.forename: Bernd F
authors.initials: BF
authors.lastname: Straub
keywords: carbene; carbenoid; copper; relativistic effects; silver