Bound Triplet Pairs in the Highest Spin States of Monovalent Metal Clusters

The chapter discusses ferromagnetic bonding in no-pair clusters of alkali and coinage metals, n+1Mn (M=Li, Na, Cu, Ag, and Au). These clusters have maximum spin and not even a single electron pair. It is shown that the bonding energy per atom, De/n, exhibits a strongly nonadditive behavior; it grows rapidly with the cluster size and converges to values of 12-19kcalmol-1 for lithium, gold, and copper. A valence bond (VB) analysis shows that this no-pair ferromagnetic (NPFM) 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+ 3M(↑↑)-, and the various excited covalent configurations (involving pz and dz2 atomic orbitals) into the fundamental covalent structure 3(M↑↑M) having s1s1 as 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 even a single electron pair.