First-principles atomic cluster calculations of the effects of trace elements on transition metal cohesion have identified different mechanisms of cohesive enhancement and decohesion. This study of Li-row interstitial atoms in a Ni6 octahedral host shows that elements which form covalent bonds with host atoms without leading to much strain (e.g. B and C) increase the cohesive strength of the host. Decohesion, a lowering of the cohesive strength of the host, is promoted by trace elements which favor the formation of ionic bonds (e.g. Li and F). In this case, decohesion occurs by mechanisms involving metal-metal bond density reduction (by charge transfer and/or Pauli exclusion) and/or by incomplete screening of the impurity nucleus. These mechanisms are identified through directly calculated restoring forces in the cluster and analysis of deformation densities which detail the charge density rearrangements in the host in the presence of Li-row interstitial atoms.