Quantifying the Electron-Donating Strength of Phosphine Ligands

This laboratory experiment for use in inorganic or organometallic chemistry allows students to examine the property of ligand electron-donor strength and its impact on a transition-metal center. Students synthesize a series of four cis-Mo(CO)4(PR3)2 complexes with phosphorus(III) ligands having a wide range of electron-donating strengths (PR3 = P(n-butyl)3, PPh3, P(OMe)3, and P(OPh)3). These complexes are characterized by FT-IR spectroscopy, with an A1 carbonyl stretch (νCOMo) used as a handle for quantifying the electron density of the molybdenum center. Using this data, students qualitatively rank the electron-donor strength of this series of ligands. In addition, students apply the Tolman electronic parameter (TEP), the quantitative analysis of ligand effects (QALE) model, and the molecular electrostatic potential (MESP) as quantitative measures of phosphine-donor ability. By developing linear correlations between these parameters and their experimental νCOMo data, students estimate the νCOMo values for additional cis-Mo(CO)4(PR3)2 complexes not synthesized in this laboratory. This combined qualitative and quantitative investigation provides students with practical insight into ligand-donor ability and its application in "tuning" the properties of a metal complex.