A molecular tool for measuring the electron-acceptor ability of ligands from crystallographic data

A method is proposed for the direct measurement of the pi-acidity of ligands coordinated to a metal center. The four-coordinate complex {[PtI2(Me-2-phen)] (Me-2-phen = 2,9-dimethyl-1,10-phenanthroline) is unique since it behaves as a molecular trap and, in the presence of a nucleophile (L), one end of it dissociates to allow the L ligand to coordinate trans to the monocoordinated diamine. The coordination of the free end of the phenanthroline is impeded by the filled d orbitals of the metal lying in the same plane as the phenanthrohne (d(yz) and d(z)(2) electrons assuming as x-axis that of the I-Pt-I vector). However, if the L ligand is a pi-acid able to remove electron charge from the yz plane, the second end of the phenanthroline will also be able to interact with platinum. A linear correlation is found between the crystallographic N-Pt-L angle 180degrees in ideal square-planar species and 143degrees in a trigonal bipyramid with a bite angle for bidentate Me-2-phen of 74degrees) and the pi-acceptor capacity of the L ligand as estimated by ab initio calculations. Alkenes and alkynes are found to be among the best pi-acids and lead to a regular trigonal-bipyramidal five-coordinate species. CO appears to be only halfway in between the best pi-acceptor ligands and those with smallest pi-acidity. P-, S-, and N-donor ligands follow in that order without significant differences between sulfides and sulfoxides or between aliphatic amines and pyridine. The very small difference between aliphatic amines and pyridine has been confirmed by investigation of the rate of exchange between the two ends for monocoordinated phenanthroline by H-1 NMR spectroscopy. The activation energy (DeltaG(double dagger)) was found to be equal, within the experimental error, for the two types of trans ligands