Linear MgCp*2 vs Bent CaCp*2 : London Dispersion, Ligand-Induced Charge Localizations, and Pseudo-Pregostic C–H···Ca Interactions

Pal, Rumpa; Mebs, Stefan; Shi, Ming W.; Jayatilaka, Dylan; Krzeszczakowska, Joanna M.; Malaspina, Lorraine A.; Wiecko, Michal; Luger, Peter; Hesse, Malte; Chen, Yu-Sheng; Beckmann, Jens; Grabowsky, Simon (2018). Linear MgCp*2 vs Bent CaCp*2 : London Dispersion, Ligand-Induced Charge Localizations, and Pseudo-Pregostic C–H···Ca Interactions. Inorganic chemistry, 57(9), pp. 4906-4920. American Chemical Society 10.1021/acs.inorgchem.7b03079

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In the family of metallocenes, MgCp*2 (Cp* = pentamethylcyclopentadienyl) exhibits a regular linear sandwich structure, whereas CaCp*2 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*2 suggests that the bending must have an intramolecular origin. Geometry optimizations with and without dispersion effects/d-type polarization functions on MCp2 and MCp*2 gas-phase complexes (M = Ca, Mg) establish that attractive methyl···methyl London dispersion interactions play a decisive role in the bending in CaCp*2. 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*2 and CaCp*2 were determined which facilitated the identification and characterization of intermolecular pseudo-pregostic interactions, C–H···Ca, in the CaCp*2 crystal structure.

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