Linear MgCp* vs Bent CaCp*: London Dispersion, Ligand-Induced Charge Localizations, and Pseudo-Pregostic C-H···Ca Interactions.

In the family of metallocenes, MgCp* (Cp* = pentamethylcyclopentadienyl) exhibits a regular linear sandwich structure, whereas CaCp* is bent in both the gas phase and solid state. Bending is typically observed for metal ions which possess a lone pair. Here, we investigate which electronic differences cause the bending in complexes lacking lone pairs at the metal atoms. The bent gas-phase geometry of CaCp* suggests that the bending must have an intramolecular origin. Geometry optimizations with and without dispersion effects/d-type polarization functions on MCp and MCp* gas-phase complexes (M = Ca, Mg) establish that attractive methyl···methyl London dispersion interactions play a decisive role in the bending in CaCp*. A sufficient polarizability of the metal to produce a shallow bending potential energy curve is a prerequisite but is not the reason for the bending. Concomitant ligand-induced charge concentrations and localizations at the metal atoms are studied in further detail, for which real-space bonding and orbital-based descriptors are used. Low-temperature crystal structures of MgCp* and CaCp* were determined which facilitated the identification and characterization of intermolecular pseudo-pregostic interactions, C-H···Ca, in the CaCp* crystal structure.