No-pair bonding in the high, spin ³Σ(u)/⁺ state of Li₂. A valence bond study of its origins

Lithium forms high-spin clusters, (n+1)Li(n) which are bonded even though there are no electron-pairs! This no-pair bonding is weak for the ³Σ(u)/⁺ state of Li₂ but becomes very significant for larger clusters reaching up to 1.8 eV for ⁷Li₆. To understand the nature of 'no-pair bonding', we performed valence bond (VB) calculations on the states of Li₂, benchmarked them against high-level MO-based calculations which account for static as well as dynamic electron correlation, and derived bonding mechanisms for the no-pair triplet state vis a vis the singlet ground state. It is shown that both the singlet-pair and no-pair bonds are bonded by covalency but differ in the mechanism of VB mixing. The singlet-pair bond is sustained by covalency augmented by Coulomb correlation of the electron pair, while the no-pair bond originates solely in the resonance energy between the repulsive fundamental triplet VB structure with the secondary VB structures. Understanding of the fundamental no-pair bond in ³Li₂ enables one to derive insight into the bonding and geometric features of no-pair (n+1)Li(n) high- spin clusters. Experimental characterization of such clusters will broaden the current conception of bonding beyond the traditional spin-pairing paradigm.