Abstract :
[en] The classes of shape of molecules or ions containing a central metal atom have been historically organized using qualitative methods such as Valence Shell Electron Pair Repulsion (VSEPR) or “Gillespie” rules. In a more theoretical manner, the Crystal Field Model describes the electronic structure and the geometry of such complexes. Even if the geometry is the basis for classification, very few direct measurements are available. The geometry of the complexes is deduced from indirect spectroscopic measurements, or, is obtained from crystallographic data.
The aim of this project is to revisit the geometric classification of central metal complexes in the absence of solvent, when only intrinsic properties of the partners play a role, among which the formal oxidation state of the cation, the steric hindrance of the ligands, their binding energies, are key factors governing the reactivity of systems essential for catalysis. All these properties can be accessed using ‘Ion Mobility’ coupled with mass spectrometry (MS) supported by action spectroscopy of trapped ion and theoretical calculations.
In the case of carboxylate ligands, the preliminary results show a linear relation between the CCS of the complex and the mass of the ligand. This smooth increase indicates the absence of steric hindrance. From energy resolved MS/MS, the V50 values are similar, as expected for ligands with similar electronic properties. Comparing the evolution of the CCS of the complexes with the CCS of the ligands alone, a predictive incremental value can be determined. To increase the electronic and steric differences, phosphine based ligands have been analyzed. We have striven with success to correlate data acquired for the phosphine ligands in the gas phase with standard steric and electronic parameters used in organometallic chemistry and homogeneous catalysis. In the case of strongly electron donating ligands, we observed an oxidation of the central cation giving a Ru3+ complexe differing only by 1 m/z from the reduced protonated Ru2+ complexe ion. The maximum arrival time differs surprisingly by 40 ms. The MS/MS spectra present new fragments showing dehydration of the p-cymene.
