10-π-Electron arenes à la carte: structure and bonding of the [E–(CnHn)–E]n−6 (E = Ca, Sr, Ba; n = 6–8) complexes

Abstract

In this paper, we provide solid evidence to show that among an overwhelming structural diversity, alkaline earth metals (Ca, Sr, Ba) have the ability to form inverted sandwich compounds with C₆H₆, C₇H₇⁺, and C₈H₈²⁺ of D_nh symmetry and general formula [E–(C_nH_n)–E]ⁿ⁻⁶ (n = 6–8) with planar 10-π-electron aromatic cores by virtue of transferring two electrons per metal atom to the ring. However, the origin of the orbital interaction between the metals and the carbon ring is quite different; while [E–(C₆H₆)–E] complexes are dominated by δ-interactions, both π- and δ-interactions are important in [E–(C₇H₇)–E]⁺ and [E–(C₈H₈)–E]²⁺ complexes.