The reaction of K2Pt(NO2)4 with dilute sulfuric acid has been investigated. At least two polynuclear complexes are formed in the reaction. The first, K2[Pt2(NO2)4(0H)2]*1½H2O, is a hydroxo-bridged dimer of Pt(II). Crystal data are space group Ibca, a = 11.879 (6) Å b = 13.094 (4) Å, c = 32.060 (11) Å, α = β = γ = 90°, V= 4987 (4) Å3, Z = 16, R = 0.065, and Rw = 0.058. The second, K5[Pt4(NO2)9O3]•3H2O, is a mixed-valence tetranuclear complex with a trinitritoplatinate(IV) unit oxo bridged to three cyclo-(μ-oxo)dinitritoplatinate(II) units. Crystal data are space group Pl, a = 9.940 (2) Å, b = 10.069 (2) Å, c = 15.206 (3) Å, α = 72.15 (l)°, β = 74.69 (1)°, γ = 72.24 (1)°, V= 1354.9 (4) Å3, Z = 2, R = 0.032, and Rw = 0.032. © 1990, American Chemical Society. All rights reserved.