Classification of the Electronic Properties of Chelating Ligands in cis-LL′Rh(CO)2 Complexes

By analogy to the Tolman electronic parameter, a ligand electronic parameter, referred to as L 2 EP, is introduced here for estimating the donating ability of chelating ligands, featuring two coordinating extremities. It is based on the average of the computed infrared stretching frequencies of CO in a series of isostructural rhodium(I)-dicarbonyl complexes, that is linearly correlated to the number x of N-heterocyclic carbene coordinating ends ( x = 0, 1, or 2). The L 2 EP values allow the design of an unified scale for the classification of the electron donation of chelating ligands, based on an ortho -phenylene bridge substituted by two coordinating extremities, which may have a different donating character. Strengths and limitations of the L 2 EP scale are illustrated for a large diversity of bidentate chelating ligands with coordinating ends ranging from extremely electron-rich phosphonium yldiides to extremely electron-poor amidiniophosphonites.