The recent discovery of an anion efficacy series for the formation and stabilization of transition-metal Ir(0)(n) nanoclusters, specifically P2W15Nb3O629- similar to SiW9Nb3O407- > C6H5O73- > [-CH2CH(CO2-)-](n)(n-) similar to OAc- similar to P3O93- similar to Cl- similar to OH--that is, polyoxoanions > citrate(3-) > other commonly employed nanocluster stabilizing anions, raises the question of what are the underlying factors behind this preferred order of stabilizers? A brief discussion of three relevant nanocluster papers in the literature, plus a concise summary of the relevant interfacial electrochemistry and surface science literature of C-3 symmetry SO42- binding to Ir(111) (as well as to Rh(111), Pt(111), Au(111) and Cu(1111)), are presented first as key background for the lattice size-matching model which follows in which tridentate anions coordinate to transition-metal nanocluster surfaces. A table of nanocluster formation and stabilization data for tridentate oxoanion stabilizers is presented, results which allow two fundamental, previously unavailable, important insights (out of 10 total insights): (i) the premier anionic stabilizers of transition-metal(0) nanoclusters present a tridentate, facial array of oxygen atoms for coordination to the metal(0) surface; and (ii) the preferred tridentate oxoanion stabilizers of nanoclusters are those that have the best match between the ligand O-O and surface Ir-Ir distances, all other factors being equal-that is, there is a previously unappreciated, geometric, anion-to-surface-metal lattice-size-matching component to the best anionic stabilizers of transition-metal nanoclusters. These are the first molecular-level insights for how the to-date premier tridentate, anionic stabilizers of transition-metal nanoclusters achieve their higher level of stabilization-a non-trivial advance since there was a lack previously of molecular-level insights into how transition-metal nanoclusters are stabilized. Four experimentally testable predictions of the C-3 symmetry, lattice size-matching model for nanocluster M(111) surfaces are presented and briefly discussed. One key prediction is that HPO42- is a heretofore unappreciated simple, effective and readily available stabilizer of Ir(0) and other transition-metal nanoclusters where there is a lattice-size match between the O-O and the surface M-M distances. Recent experimental evidence is summarized revealing that this prediction is, in fact, trite-that is, the third key, new finding of this work is (iii) the first rational design of a new nanocluster stabilizer, HPO42-, one shown to be as good a stabilizer as the common nanocluster stabilizer citrate(3-). The C-3 symmetry, lattice size-matching model is significant in seven additional ways which are detailed in the text and summary which follows. (C) 2003 Elsevier B.V. All rights reserved.