Introduction: N-Heterocyclic carbenes, also refered to as NHC, are stable singlet carbenes that can act as excellent two electron donor ligands towards almost any element in the periodic table. They coordinate strongly to late transition metals and heavy main group elements, but are also known to bind to early transition metals and the lanthanoids. However, there are alternative methods to prepare air- and moisture-stable species that can be used to transfer carbenes to other metals. In particular this involves the preparation of imidazolium salts and reacting it with Ag2O to form robust silver complexes.(1)
Discussion:
(1)
Formation of imidazolinium chlorides from corresponding aniline
C2H4Cl2 (l)+ 2C6H7N (l) + C7H16O3 (l) 2C21H25ClN2

Synthesis of (SIPr)AgCl
2SIPr.HCl (l)+ AgO(s) → 2 (SIPr)AgCl(l) + H2O(l)

(2)
The NHC ligand is significantly more tightly bonded than the phosphine ligand as a consequence of the larger energetic contributions of the σ-donation, π-back-donation. Back-bonding in carbonyl and phosphine metal complexes influence the bond strength. Donation of electrons from the filled π-orbital or lone electron pair orbital of the ligand into an empty orbital of the metal (donor–acceptor bond), together with release (back donation) of electrons from an nd orbital of the metal (which is of π-symmetry with respect to the metal–ligand axis) into the empty π*-antibonding orbital of the ligand. (2) Singlet carbenes are also highly basic and are powerful electron donors. These properties make carbenes exceptionally good ligands for a wide variety of metals. (2)
Figure 1: Diagram showing NHC orbitals interacting with transition metal orbitals
(3)
Substituents capable of π donation, such as N, P, O and F, favour the singlet state over the triplet state. The N-herterocyclic carbenes contains singlet carbeneic carbon centers that are bonded to two nitrogen atoms. A “free carbine” (a bare carbon atom) is in the triplet state. Figure 2: Singlet and triplet free carbenes.
(4)

Resonance structures for a N-heterocyclic carbine
References:
1. Masuda, Jason D. Structure and Bodning of the Main Group Elements. Halifax : Saint Mary's University, 2010.
2. A Three-Step Laboratory Sequence to Prepare a Cabene Complex of Silver(I) Cloride. Canal, P John, Ramnial, Taramatee and Langlois, Lisa D. 5, s.l. : Journal of Chemical Education, Vol. 85.