A systematic density functional study of the structure and bonding in the alkali-metal pentadienyl complexes C5H7E (E = Li-Cs) and their analogues derived from the 2,4-dimethylpentadienyl ligand is performed. The bonding in these structures has been analyzed in some detail with reference to molecular orbital analysis, and energy partition analysis, obtained by density functional calculations. An energy decomposition analysis indicates that the electrostatic interaction is the main factor to be considered in the stabilization of the gas-phase complexes we have studied. The stability of the U-shaped minimum energy structure decreases (the potential energy surface becomes more shallow) as the metal atom gets larger. We trace this behavior to a weakening of the metal–ligand binding due to the increasing diffuseness of the metal p orbitals on going down group 1. A significant pyramidalization at the terminal carbons in the coordinate U-shaped structure correlates with the strength of the metal–ligand binding. Initial results for the structural preferences of the complexes in solution for the lithium pentadienyl complex are examined in view of contrary experimental data. There still remains plenty of work to be done in modeling metal complexes in solution, and we suggest a way forward.

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Copyright © 2008 American Chemical Society. This article first appeared in Organometallics 27:5 (2008), 827-833.

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