**2. Methods for assessing electronic and steric parameters of NHCs**

#### **2.1 IR spectroscopy: Tolman electronic parameters (TEP)**

The most extensively used method for the determination of electronic properties of NHCs is based on the Tolman electronic parameters devised by Tolman in 1977, specifically for phosphines [24]. This technique uses tetrahedral Ni(CO)3L as model complex [25], where L is the ligand whose donor-acceptor abilities are under investigation. Carbonyl being a good π-acceptor has strong metal-to-carbonyl (dπ-pπ) back bonding. The introduction of ligands (L) with strong electron-donor properties will enhance the electron density around the metal (Ni) center in the Ni(CO)3L complex.

**Figure 4.** *Determination of TEP using IR spectroscopy [25].*

Thus, by comparing the stretching frequencies of the carbonyl (CO) unit, various ligands (L) of interest were analyzed by their σ-donor properties. Ligands (L) with good σ-donor ability increase the metal-to-carbonyl back bonding. It results in stronger metal-ligand (Ni-L) bonding. Thus, it indirectly reduces the bond order of the carbonyl unit, and it will be reflected in the carbonyl IR stretching frequency (**Figure 4**). Nolan et al. were the first to systematically determine the TEP of NHCs using [Ni(CO)3(NHC)] as model complex. The critical limitation of this method is the preparation of a variety of nickel complexes using different NHC ligands. This synthesis requires the handling of NHCs with low boiling and toxic Ni(CO)4. Moreover, the TEP values are within a small window of 10 cm<sup>1</sup> and hence require high-resolution IR spectrophotometers. Any anomalies inherent to the IR measurement technique make this method more challenging. TEP depends on the steric factors since the steric can influence the complexes geometry, thereby the overlapping orbitals affect the TEP value. To overcome few of these shortcomings with respect to synthesis of analytical probes, Crabtree et al. [26] in 2003 reported the use of [(NHC)M(CO)2Cl], (M = Ir or Rh) complexes as the probe for determining the TEP. In 2008, Nolan et al. reported an extensive study on the Ir complexes for TEP determination of NHCs [27]. Compared with the earlier model complexes, an average of two carbonyl stretching vibration are taken into account due to *cis* arrangement of the carbonyl ligands in the [(NHC)M(CO)2Cl], (M = Ir or Rh) complex. Thus, the IR spectroscopy was found to be an useful tool to assess the Tolman electronic parameters (TEP). TEP of selected NHCs are given in **Table 1**.
