Reaction of [PtMe2(NN)] (1, NN = 1,10-phenanthroline or 2,2'-bipyridyl) with ¿-PrX under inert atmosphere gives [PtXMe2-¿-Pr(NN)] (2, X = Br; 3, X = I) but, in the presence of oxygen, t'-Prl reacts to give a mixture of 3, [PtIMe2(00-t-Pr) (NN) ], 4, and [PtI2Me2(NN)], 5. The reaction with ¿-PrBr occurs by the S>j2 mechanism, but with ¿-PrI the mechanism is largely free radical in nature. Oxygen acts as a reagent to trap the intermediate ¿-Pr• radicals, is incorporated into the product 4, and acts as an inhibitor of the free radical chain reaction. The reactions with ¿-PrI are greatly accelerated by light; in the dark and in the presence of free radical inhibitors such as oxygen, the Sn2 mechanism is thought to operate to a minor extent. t-BuI reacts with [PtMe2(NN)] in the presence of air to give [PtIMe2(00-t-Bu)(NN)]. These are the first (alkylperoxo)platinum(IV) complexes. The structure of [PtIMe2(00-¿-Pr)(NN)] was determined by X-ray crystallography. Crystals are monoclinic, space group 2 /n, with four molecules in a cell of dimensions a = 9.946 (2) k,b = 11.340 (3) Á, c = 16.374 (2) Á, and ß = 94.55 (1)°. The structure was solved by the heavy-atom method and refined by full-matrix least-squares calculations with anisotropic thermal parameters for the non-hydrogen atoms. At convergence, R = 0.0230 and Rv = 0.0242 for 1402 observed data. The Pt atom has octahedral coordination, with trans oxygen and iodine atoms. Principal dimensions are Pt-O(l) = 2.032 (6) Á, Pt-I = 2.624 (1) A, Pt-Me = 2.048 and 2.067 (8) Á, Pt-N = 2.164 and 2.175 (7) A, 0(l)-0(2) = 1.465 (9) A, and Pt-0(l)-0(2) = 110.2 (4)°.