Classification of metallic complexes by ion mobility mass spectrometry

The shape of molecules or ions containing a central metal atom have been historically categorized 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.
Ion mobility hyphenated to mass spectrometry allows the determination of the electrical mobility in gas phase which is correlated to the collisional cross section (CCS), the charge and the reduced mass of the ions. CCS is a physical property which represents the rotationally averaged collision cross section area (of the corresponding spherical body in the hard sphere approximation model) but there is no intuitive correlations between the CCS and the shape of an ion. Preliminary results of our recent work concerning iron and copper complexes with halogens and carboxylates anions suggest that the ratio between CCS and mass allows a classification of complexes depending on their tridimensional structure (i.e. tetrahedral, planar or linear structures) and more interestingly the ligands density. The geometries experimentally obtained for metallic complexes are supported by theoretical structure optimization using Hartree-Fock and Density Functional Theory (DFT) calculations. In perspective, this strategy should check the validity of VSEPR or Crystal Field models to describe the geometry/shape of complexes having a specific central metal atom in absence of solvent. The aim of this short presentation is to introduce the strategy of metallic complexes categorization in function of their geometry and the effect of the ligand density on their ion mobility value.