Immersion of an ordered Au(111) electrode in a 5 similar to 50 muM haloplatinate solution for 1 min results in spontaneous and irreversible adsorption of a submonolayer to a monolayer Pt complex. In situ STM imaging reveals the formation of highly ordered molecular adlayers of Au(111) - (root7 x root7)R19.1degrees - PtCl42- and PtBr42- (theta = 0.143) in a solution containing 50 muM of each complex. Dosing in a dilute PtBr42- solution (5 pM) produces another highly ordered structure, identified as Au(111) - (2root19 x 2root19)R23degrees 7 PtBr42-+ 6 Br. Atomic resolution STM reveals pinwheel features of the planar PtX42- adsorbates (X Cl, Br, 1). The molecular adlayers of PtI62- are always disordered, irrespective of the dosing conditions. The reduction potentials of these molecular overlayers to produce the deposit of metallic Pt decrease in the order of PtCl42- > PtBr42- > PtI62-. The Pt deposit exists in the form of nanoparticles with an average diameter of 3.0 nm and height of 0.46 nm, and they are uniformly distributed on the Au(I 11) substrate. The electrocatalysis toward the oxygen reduction of Pt particles on Au(111) is examined with a hanging-meniscus rotating disk technique. The Pt particles prepared with the precursors of chloro- and bromoplatinate are active in catalyzing oxygen reduction, whereas those from iodoplatinate are inert. This result correlates with the strength of surface bond between halide and An or Pt metal. Iodine adatoms evidently block the active sites on Pt clusters.