Spontaneously adsorbed monolayers of [Os(bpy)(2)4bptCl](PF6) have been formed on platinum microelectrodes. bpy is 2,2'-bipyridyl, and 4bpt is 3,5-bis(pyridin-4-yl)-1,2,4-triazole. These monolayers exhibit well-defined, almost ideal electrochemical responses over a wide range of voltammetric scan rates and in a wide range of electrolytic solutions. The surface coverage of these monolayer films is consistent with that expected for a close-packed monolayer, in which the area of occupation is governed by the area of the redox-active headgroup rather than by the bridging ligand. The differential capacitance of the monolayer-modified interface is 18 +/- 3 muF cm(-2) compared to 35 +/- 3 muF cm(-2) for an unmodified surface. Consistent with the observation that the formal potential of the Os2+/3+ process shifts by less than 30 mV upon immobilization, these data suggest that the monolayers are well solvated. The dependence of the differential capacitance on solution pH reveals that the pK(a) of the triazole bridge within the monolayer, 8.9 +/- 0.3, is indistinguishable from that found in solution. Chronoamperometry, conducted on a nanosecond time scale, reveals that the redox switching mechanism involves hole rather than electron transfer. Significantly, upon protonation of the 4bpt bridging ligand, the standard heterogeneous hole transfer rate constant decreases from 1.60 to 0.2 x 10(6) s(-1) for the reduction mechanism and from 2.7 to 0.05 x 10(6) s(-1) for the oxidation process. These observations are consistent with the redox mechanism occurring via a hole-transfer process, the rate of which depends on the energy difference between the metal d pi orbitals and the HOMO of the bridge. Protonation of the bridging ligand increases this energy gap, resulting in an overall decrease in the rate of the redox reaction.