On the Bonding Nature in the Crystalline Tri-Thorium Cluster: Core-Shell Syngenetic σ-Aromaticity.

A unique thorium-thorium bond was observed in the crystalline tri-thorium cluster [{Th(η 8 -C 8 H 8 )(μ 3 -Cl) 2 } 3 {K(THF) 2 } 2 ] ∞ , though the claim of σ-aromaticity for Th 3 bond has been questioned. Herein, a new type of core-shell syngenetic bonding model is proposed to describe the stability of this tri-thorium cluster. The model involves a 3c-2e bond in the Th 3 core and a multicentered (ThCl 2 ) 3 charge-shift bond with 12 electrons scattering along the outer shell. To differentiate the strengths of the 3c-2e bond and the charge-shift bond, the block-localized wavefunction (BLW) method which falls into the ab initio valence bond (VB) theory is employed to construct a strictly core/shell localized state and its contributing covalent resonance structure for the Th 3 core bond. By comparing with the σ-aromatic H 3 + and nonaromatic Li 3 + , the computed resonance energies and extra cyclic resonance energies confirm that this Th 3 core bond is truly delocalized and σ-aromatic.