Structure and Bonding of Alkali-Metal Pentalenides

The lowest energy isomers of alkali-metal pentalenides, E2C8H6 (E = Li, Na, K, Rb, Cs), are inverted sandwiches. Along Li to Cs, the location of the E atoms shifts toward the points over the center of the pentalene moiety even in the presence of solvent molecules such as dimethoxyethane. Adaptive natural density partitioning analysis reveals the equivalent 10 π-bonding frameworks in the C8H62– and E2C8H6 systems. The stability of these complexes practically originates from the electrostatic interaction (84–92%) between C8H62– and [E···E]2+. While the sharp drop in interaction energy in Na complex, in comparison to that in the Li analogue, is due to the lower contribution from both electrostatic (by 31.6 kcal mol–1) and orbitalic (by 48.1 kcal mol–1) terms, for the rest of the complexes the obtained trend of interaction energy originates from the reduced ionic contacts. Although the orbital interaction is less important in these complexes, it plays an important role in deciding their geometries. The obtained geometrical change along Li to Cs is a consequence of the participation of the d orbitals in the heavier analogues.