The Huynh  Electronic Parameter (HEP) was first introduced in 2009 as a modern method to evaluate the electronic properties of both Werner-type and organometallic ligands on a unified scale using ¹³C NMR spectroscopy. HEP essentially evaluates the influence of a trans-ligand L on the ⁱPr₂-bimy carbene resonance in complexes of general formula trans-[PdBr₂(ⁱPr₂-bimy)L] (ⁱPr₂-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene). The HEP value for a given ligand L is the ¹³C NMR chemical of the ⁱPr₂-bimy carbene resonance measured in ppm, whereby a stronger L donor induces a relative downfield shift of this signal compared to a weaker donor. Since backdonation from the Lewis acidic Pd^II centre is insignificant, we are primarily determining the relative σ–donor strength of a ligand L. Figure 1 illustrates the working principle of HEP and the experimentally observed trends for the ⁱPr₂-bimy carbene signal, i.e. HEP value in ppm, induced by the donating abilities of ligands.

Figure 1. The principle of the Huynh Electronic Parameter (HEP).

"Strong donors increase the electron density of a complex and this shielding effect will lead to a highfield shift." This statement is one of the most common misconceptions about trends in chemical shifts that is a result of oversimplification of concepts taught on NMR spectroscopy in many organic chemistry courses. For a proper understanding, one must appreciate that chemical shifts are a result of three prime factors (shielding terms), i.e. diamagnetic term σ_d, paramagnetic term σ_p and neighboring group anisotropy σ_n. Unfortunately, many lecturers only focus on simple ¹H NMR spectrosocopy, where the paramagnetic term σ_p plays a minor role. In any other nuclei, however, it can even become the main contributor to the chemical shift. Knowledge of the paramagnetic shielding term σ_p is essential for a deep understanding of HEP. However, a detailed description of these is certainly beyond the scope of this small paragraph, and proper literature material should be referred to instead. Here, we will just mention some brief statements and known facts that can help one understand the trends observed in our ¹³C NMR based electronic parameter. These are also illustrated in Figure 1 for a better understanding. 

Overall, the following statements can be made:

1. Free NHCs have very downfield carbene NMR resonances of >200 ppm.
2. Upon metal-coordination, the carbene NMR signal shifts significantly upfield.
3. A stronger trans-ligand L in our complex probes weakens the Pd–C_NHC bond more than a weaker donor, i.e. trans influence.
4. The weakening of the Pd–C_NHC is generally accompanied by a bond elongation, i.e. trans influence.
5. Stronger trans-ligands will thus increase the "free-carbene" character of the  ⁱPr₂-bimy reporter ligand, which results in a downfield shift of its ¹³C_carbene NMR signal.
6. In an extreme (fictive) case, a super-strongly donating ligand would lead to dissociation of the  ⁱPr₂-bimy reporter ligand, which chemical shift would be >200 ppm.

Bidentate ligands are known to impart greater stability to complexes due to the chelate-effect. Despite their importance in coordination chemistry, it is surprising that electronic parameters for bidentate ligands are scarce. Nevertheless, it is obvious that the evaluation of such species will be more challenging due to increased possibility of interfering interactions (e.g. sterics, isomerizations). In order to evaluate the donating abilities of bidentate ligands in a systematic way, we have extended HEP to HEP2 in 2014 using the same underlying principle. The initial focus was set on symmetrical chelators, for which isomerization processes do not play any role. Since its inception, HEP2 has also been applied to a few unsymmetrcial bidentate ligands with two different donor atoms (see HEP2 scales).

HEP values have been determined and published for >70 Werner-type and organometallic ligands and their donating abilities can be compared to each other. A convenient comparison of various ligands can be done by plotting their HEP values on a ¹³C NMR scale. This has been done for various types of donors and are depicted below.

N-donors

P-donors

NHC donors

Non-classical NHCs