Topological studies of the charge density of some group 2 metallocenes M(eta(5)-C5H5)(2) (M=Mg or Ca)

Ab initio quantum chemical calculations using both the Hartree-Fock and the B3LYP density functional theory methods have been performed for the group 2 metallocenes M(eta(5)-C5H5)(2) (M = Mg or Ca). The topology of the calculated charge density (rho) and its Laplacian (del(2) rho) have been analysed using a new critical point search algorithm in order to understand why Mg(eta(5)-C5H5)(2) is linear while Ca(eta(5)-C5H5)(2) is bent. The Laplacian of the core of the Ca atom in Ca(eta(5)-C5H5)(2) is perturbed by the polarizing field of the cyclopentadienyl ligands and the bent geometry is a consequence of the interactions between the distorted core and the ligand atoms. In the case of Mg(eta(5)-C5H5)(2), charge concentration maxima in the Mg core occur along the vectors connecting the metal to the centroids of the cyclopentadienyl ligands irrespective of whether or not the molecule is linear, and the preferred geometry is linear as expected. The results of these calculations demonstrate that the geometries of the group 2 metallocenes can be understood in terms of the repulsive interactions between the ligands and between the ligands and the distorted core of the metal atom.