This work tests the hypothesis that tridentate oxoanions are especially effective stabilizers of transition-metal nanoclusters when the O-O distance of the anions matches closely the M-M (M = metal) distance atop the nanocluster surface. Specifically, we test the hypothesis that HPO42- with its 2.5 Angstrom O-O distance is a very simple, effective, but previously unrecognized anion for the stabilization of transition-metal(O) nanoclusters such as those of Ir(O), where the Ir-Ir surface distance is ca. 2.6-2.7 Angstrom. This hypothesis is tested by the five criteria we recently developed. These criteria emphasize the ability of a given nanocluster-stabilizing anion to allow the formation of highly kinetically controlled, near-monodisperse (less than or equal to+/-15%) size distributions of nanoclusters and then to allow isolable and fully redissolvable nanoclusters that exhibit, once redispersed into solution, good catalytic activity and long catalytic lifetimes. The previously unknown precursor complex {[Bu4N] [(1,5-COD)Ir.HPO4]}(n), 1, was prepared and shown to be a preferred precursor for the reproducible formation of hydrogen phosphate- and tetrabutylammonium-stabilized transition-metal lr(O) nanoclusters. The nanocluster formation reaction was shown to follow the slow continuous nucleation (A --> B, rate constant k(1)) followed by fast autocatalytic surface growth (A + B --> 2B, rate constant k(2)) mechanism uncovered previously; this finding was then exploited by showing that nanocluster size control could be achieved as expected by adding excess HPO42- to lower the k(2)/k(1) ratio, resulting in the formation of smaller nanoclusters. A relatively rare experimental demonstration of the balanced reaction for nanocluster formation is also provided. Proton Sponge [i.e., 1,8-bis(dimethylamino)naphthalene] is shown to be a preferred scavenger of the 1 equiv of H+ byproduct formed from the H-2 reduction of the (1,5-COD)Ir(l)+ moiety in the nanocluster precursor to Ir(O) plus H+; positive effects of Proton Sponge on the resultant nanocluster catalytic lifetime are also demonstrated. Transmission electron microscopy (TEM) of the postcatalysis nanoclusters shows that agglomeration is a catalysis-inhibiting deactivation reaction. Overall, the results show that HPO42- is an effective anion for the formation, and then stabilization, of lr(O) transition-metal nanoclusters in acetone and with Bu4N+ countercations. More specifically, HPO42- rates alongside citrate(3-) in the developing series of anion efficacy for nanocluster formation, stabilization and catalytic activity: polyoxoanions > HPO42- similar to citrate(3-) > other commonly employed nanocluster-stabilizing anions. Since a reasonable match between the tridentate O-O distance in HPO42- and the M-M distances is present for the metals Fe, Co, Ni, Ru, Rh Ir, Pd, Re, Os, and Pt [i.e., the lattice size-matching criterion is fulfilled; Ozkar, S.; Finke, R. G. Coord. Chem. Rev. 2003 (submitted for publication)], our results imply that HPO42- merits consideration for nanocluster synthesis and stabilization any time M(O) nanoclusters of the above list of metals are planned. The additional advantages of HPO42- are also presented and briefly discussed, namely, its thermal robustness, its high resistance to reduction or oxidation, its valuable