Introduction of modified electronic parameters - Searching for a unified ligand properties scale through the electrophilicity index concept

In order to quantify the contribution of mainly, but not only, dim-tine ligands to the electronic properties of the corresponding complexes, two novel indices, namely modified electronic parameters (ModEP) and electrophilicity, omega, are introduced and discussed in this report. First, we introduce the concept of ModEP, a novel scale relating to the electronic properties of diimine ligands. Mainly on the basis of well-characterized M(diimine)(dithiolato) complexes (M = Pt, Pd, or Ni), we derive the contribution of the diimine ligand to the reduction potentials of the complexes, and incorporating its contribution to the main charge-transfer band of the UV/Vis spectra, we obtain our ModEP values. The applicability of the introduced scale to several other classes of diimine compounds (W, Re, and Ru) is then tested. The performance of our scale in predicting the values of the reduction potentials and the position of the charge-transfer bands is very satisfactory. With this approach, some interesting aspects of ligand coordination were revealed. Next, we employ the recently introduced electrophilicity index omega, in order to correlate our ModEP scale with various other empirical scales that are already known and extensively used. This permits us to unify most known scales, which were primarily derived for different types of ligands (e.g. ModEP for diimines, Tolman's for phosphanes). The proposed method of omega calculation (at the sermempirical AM1 level) avoids any experimental limitation and can be employed even for ligands that are not yet experimentally available, whereas its correlation to the known scales would immediately provide the properties of the compound. This supports the idea that the electrophilicity index could become a powerful tool in coordination and organometallic chemistry. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)