Transition Metal Complexes

Abstract

Phosphanes possess an electron lone pair with which they can bond to a transition metal. As this bonding interaction results in the transfer of electron density from phosphorus to the metal atom, we would expect a downfi eld shift in the 31 P-NMR spectrum relative to the value for the free ligand that depends both on the s-donor strength of the phosphane, and the Lewis acidity of the transition metal fragment. Since most phosphane ligands also show some p-acceptor ability, coordination to a transition metal can result in p-backbonding that would increase the electron density on phosphorus, and thus result in an upfi eld shift of the signal in the 31 P-NMR spectrum. Depending on the magnitude of the downfi eld shift due to the s-donicity, and the upfi eld shift due to the p-acceptor strength of the phosphane, either a net downfi eld or net upfi eld shift is observed upon coordination of a phosphane to a transition metal. How can we estimate the sign and the magnitude of this expected chemical shift? The chemical shift difference observed upon coordination to a transition metal is known as the coordination chemical shift, and defi ned as: coordination chemical shift: Δd = d P (complex) − d P (ligand) Tricoordinate phosphorus ligands are usually strong s-donors, resulting in down-fi eld coordination shifts of around d P = 20-70 ppm in the absence of appreciable p-acceptor strength, and depending on the transition metal fragment they coordinate to. Exactly here lies the problem, and we will have to make an educated guess about the p-acceptor strength of our phosphorus ligand and the electronic properties of the transition metal fragment in question. We can do this by drawing on the information already available to us. We know from our discussion of electronic properties of phosphorus ligands (see box story in chapter 5) that the coordination chemical shift is likely to decrease in the following order: Δ d (PR 3) > Δd (PAr 3) > Δd (P(OR) 3) We deduce from our knowledge concerning the electronic properties of the d-block metals that the coordination chemical shift is likely to decrease going down the group, 83 O. Kühl, Phosphorus-31 NMR Spectroscopy,