Metal ions are commonly used as activators to improve the anionic collector flotation of oxide minerals with an accepted activation mechanism that the prior addition of metal ions can adsorb onto the oxide mineral surface to increase the number of active sites for subsequent attachment of a collector. The improved flotation of cassiterite (or scheelite) with the use of lead nitrate (LN) as the activator and benzohydroxamic acid (BHA) as the collector is a typical example. However, our recent research showed that a soluble metal-organic complex (lead-BHA complex) produced by pre-mixing LN and BHA further improved the collecting ability and selectivity in the flotation of cassiterite and for that, the existing activation mechanism cannot completely explain this further improved performance. We believe that the lead ion and its hydroxide species can directly interact with water molecules to form Pb(H2O)(n)(2+) and Pb(OH)(H2O)(n)(+), complexes in water solution, and those bonded water molecules will in turn affect the interaction of (hydroxide) lead ion with the cassiterite surface and the BHA. In this work, solution chemistry calculation was used to identify the major active components of the lead-BHA complex in water solution at different pH levels, and based on density functional theory, the adsorption energies of different activation mechanisms were calculated via accurate first-principle calculations, indicating a much stronger interaction of the adsorption of HO-Pb-BHA complex on cassiterite surface (-48.11 kcal/mol) compared with that of the sequential adsorption of Pb(OH)(H2O)(5) and BHA anion (-13.29 kcal/mol) at optimal flotation pH 8-9. That is, the difference in two activation systems can be explained based on thermodynamics. With such, a new hypothetic activation model was proposed to explain the improved performance of soluble lead-BHA complex and then, the proposed model was verified by the flotation experiment and adsorption test results. This work can help enrich the activation theory of metal ions in anionic collector flotation of minerals. (C) 2018 Elsevier Inc. All rights reserved.