p-Acidity and p-basicity of N-heterocyclic carbene ligands. A computational assessment Heiko Jacobsen a, * ,1 , Andrea Correa b , Chiara Costabile b , Luigi Cavallo b a KemKom, Libellenweg 2, 25917 Leck, Germany b Department of Chemistry, Universita ` di Salerno, Via Salvador Allende, Baronissi (SA) I-84081, Italy Received 31 October 2005; received in revised form 12 January 2006; accepted 13 January 2006 Available online 23 February 2006 Abstract In this contribution we report on a systematic analysis of the bond between transition metals and N-heterocyclic carbene (NHC) ligands. We compare the r and p-contributions in a series of complexes in which the formal d-electron count ranges from d 0 to d 10 . Our results confirm the currently accepted idea that NHC are not pure r-donors. In the series of complexes examined here p-contribu- tion is 10% at least. Moreover, remarkable metal-to-ligand backdonation occurs also for d 0 complexes, and many systems present a sub- stantial ligand-to-metal p donation. Ó 2006 Elsevier B.V. All rights reserved. Keywords: N-Heterocylic ligands; Carbene ligands; Transition metal complexes; DFT calculations 1. Introduction Now, almost 15 years after Arduengo and co-workers have isolated a stable cyclic diamino carbene in the form of an imidazol-2-ylidene derivative [1] and have brought to conclusion Wanzlick’s quest for stable carbenes actively pursued during the 1960s [2], N-heterocyclic carbenes (NHC) are emerging as a new class of versatile ancillary ligands in transition metal (TM) chemistry. The successful use of NHCs introduced a new and powerful concept in catalysis [3–5], since these compounds represent a valid alternative to the widely used phosphine ligands. Organo- metallic complexes containing NHC ligands are effectively used in ruthenium catalyzed olefin metathesis [6–10], irid- ium-catalyzed hydrogenation and hydrogen transfer [11– 13], platinum catalyzed hydrosilylation [14], and palladium catalyzed C–C coupling reactions [15–18], to name impor- tant, representative examples. Although NHC ligands are acquiring a widespread usage, our understanding of the bonding of NHC ligands to transition metals is still limited, and constitutes a topic of ongoing research activity. The seminal theoretical stud- ies of N-heterocyclic carbenes that appeared five years after Arduengo’s initial report dealt with the issues of charge dis- tributions and aromaticity [19,20], and supported the gen- eral picture of bonding properties of the NHC ligand that evolved during the first years of TM–NHC chemistry. Imi- dazol-2-ylidene was first suggested to bind to a transition metal center through r-donation only [21], a notion that was further corroborated by calculations on TM complexes of the type ClM  NHC, M = Cu, Ag, Au [22]. However, a significant amount of p-interaction between group 11 metals and NHC ligands was recently proposed on the basis of structural data [23,24]. This proposal was further supported in theoretical calculations and the p-back-bond- ing interactions for a diaminocarbene model compound was estimated to contribute to approximately 15–30% of the complexes overall orbital interaction energy [25]. In a recent computational study, in which the bonding in 0022-328X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2006.01.026 * Corresponding author. E-mail addresses: jacobsen@kemkom.com (H. Jacobsen), lcavallo@ unisa.it (L. Cavallo). 1 Present address: Department of Chemistry, Tulane University, Freret St., New Orleans, LA 70118, USA. www.elsevier.com/locate/jorganchem Journal of Organometallic Chemistry 691 (2006) 4350–4358