The mechanism for the oxidation of 3'-dGMP by [PtCl4(dach)] (dach = diaminocyclohexane) in the presence of [PtCl2(dach)] has been investigated using density functional theory. We find that the initial complexation, i.e., the formation of [PtCl3(dach)(3'-dGMP)], is greatly assisted by the reaction of the encounter pair [PtCl2(dach)center dot center dot center dot 3'-dGMP] with [PtCl4(dach)], leading to migration of an axial chlorine ligand from platinum(IV) to platinum(II). A dinuclear platinum(II)/platinum(IV) intermediate could not be found, but the reaction is predicted to pass through a platinum(III)/platinum(III) transition structure. A cyclization process, i.e., C8-O bond formation, from [PtCl3(dach)(3'-dGMP)] occurs through an intriguing phosphate-water-assisted deprotonation reaction, analogous to the opposite of a proton shuttle mechanism. Followed by this, the guanine moiety is oxidized via dissociation of the Pt-IV-Cl-ax bond, and the cyclic ether product is finally formed after deprotonation. We have provided rationalizations, including molecular orbital explanations, for the key steps in the process. Our results help to explain the effect of [PtCl4(dach)] on the complexation step and the effect of a strong hydroxide base on the cyclization reaction. The overall reaction cycle is intricate and involves autocatalysis by a platinum(II) species.