Bound triplet pairs in the highest spin states of coinage metal clusters

The work discusses bonding in coinage metal clusters, ⁿ⁺¹M _n (M = Cu, Ag, Au), that have maximum spin without a single electron pair. It is shown that the bonding energy per atom, D_e/n, exhibits a strong nonadditive behavior; it grows rapidly with the cluster size and converges to values as large as 16-19 kcal/mol for Au and Cu. A valence bond (VB) analysis shows that this no-pair ferromagnetic bonding arises from bound triplet electron pairs that spread over all the close neighbors of a given atom in the clusters. The bound triplet pair owes its stabilization to the resonance energy provided by the mixing of the local ionic configurations, ³M(↑ ↑)^-M⁺ and M ^{+ 3}M(↑ ↑)^-, and by the various excited covalent configurations (involving p_z and d_z² atomic orbitals) into the fundamental covalent structure ³(M↑ ↑M) with a s¹s¹ electronic configuration. The VB model shows that a weak interaction in the dimer can become a remarkably strong binding force that holds together monovalent atoms without a single electron pair.