*2.3.4 NHC adducts-bonding*

The researchers have investigated the bonding between carbene center and metals/non-metals over the past two decades [8, 15]. The main feature of this bonding is the donation of carbene lone pair of electrons into empty sigma orbital of metal/non-metal. It is evident in the graphical representations (**Figure 9**) of NHCmetal complexes (53, 54, and 55) that inspite of a double bond usually a single bond is shown. Therefore, in the deficiency of steric restrictions or chelation, the bond between metal and NHC can rotate easily. The π-orbital in the NHC's complexes and metal is restricted within the NHC ring so indicated by a curved line between the heteroatoms of the ring.

Metal donates its electrons in the carbene's vacant p-orbital (LUMO) of carbene. The carbene can also donate electrons in the vacant π-orbitals of metal. The significance of each factor depends on the nature and geometry of another ligand on the complex and, most importantly nature of carbene itself. Using spectroscopic methods π-accepting ability of different NHCs can be measured and quantified [54, 79].
