text: Several other scales have been proposed for the ranking of the donor properties of ligands.  The HEP scale ranks ligands on the basis of the C NMR shift of a reference ligand.  Lever's electronic parameter ranking is related to the Ru(II/III) couple. Another scale evaluated ligands on the basis of the redox couples of [Cr(CO)L].
  In a treatment akin to the TEP analysis, the donor properties of N-heterocyclic carbene (NHC) ligands have been ranked according to IR data recorded on cis-[RhCl(NHC)(CO)] complexes.