The mechanism of oxidation by O2 of (dpms)Pt IIMe(OH2) (1) and (dpms)PtIIMe(OH)- (2) [dpms = di(2-pyridyl)methanesulfonate] in water in the pH range of 4-14 at 21 °C was explored using kinetic and isotopic labeling experiments. At pH ≤8, the reaction leads to a C1-symmetric monomethyl Pt IV complex (dpms)PtIVMe(OH)2 (5) with high selectivity â‰¥ 97%; the reaction rate is first-order in [PtIIMe] and fastest at pH 8.0. This behavior was accounted for by assuming that (i) the O2 activation at the PtII center to form a PtIV hydroperoxo species 4 is the reaction rate-limiting step and (ii) the anionic complex 2 is more reactive toward O2 than neutral complex 1 (pKa = 8.15 ± 0.02). At pH ≥10, the oxidation is inhibited by OH- ions; the reaction order in [Pt IIMe] changes to 2, consistent with a change of the rate-limiting step, which now involves oxidation of complex 2 by PtIV hydroperoxide 4. At pH ≥12, formation of a C1-symmetric dimethyl complex 6, (dpms)PtIVMe2(OH), along with [(dpms)Pt II(OH)2]- (7) becomes the dominant reaction pathway (50-70% selectivity). This change in the product distribution is explained by the formation of a Cs-symmetric intermediate (dpms)PtIVMe(OH)2 (8), a good methylating agent. The secondary deuterium kinetic isotope effect in the reaction leading to complex 6 is negligible; kH/kD = 0.98 ± 0.02. This observation and experiments with a radical scavenger TEMPO do not support a homolytic mechanism. A SN2 mechanism was proposed for the formation of complex 6 that involves complex 2 as a nucleophile and intermediate 8 as an electrophile.