Reaction of 2-Li-4-MeC6H3AsPh2 with [PtCl2(SEt2)(2)] gives two isomeric dinuclear platinum(II) complexes, one containing a half-lantern structure with two chelating and two bridging C6H3-5-Me-2-AsPh2 ligands, [Pt-2(kappa(2)As,C-C6H3-5-Me-2-AsPh2)(2)(mu-kappaAs,kappaC-C6H3-5-Me-2-A sPh2)(4)], and the other, a full-lantern with four bridging C6H3-5-Me-2-AsPh2 ligands, [Pt-2(mu-kappaAs,kappaC-C6H3-5-Me-2-AsPh2)(4)]. The lantern structure of the latter is retained in the dihalodiplatinum(III) complexes that are formed by oxidative addition of chlorine, bromine, or iodine to the isomeric mixture. The dichloro derivative undergoes metathesis reactions with silver or sodium salts, yielding the corresponding cyano, thiocyanato, cyanato, and fluoro species. The structures of all complexes have been determined by single-crystal X-ray analysis. The oxidative addition products have Pt-Pt distances in the range 2.65-2.79 Angstrom (cf. 2.89 Angstrom in the lantern diplatinum(II) structure), consistent with the presence of a Pt-Pt bond. Electrochemical data lead to the conclusion that an initial rapid one-electron process and subsequent chemical reactions produce the dihalodiplatinum(III) lantern structure when mixtures of the lantern and half-lantern complexes are oxidized by halogens. The electrochemical data also explain why chemical reduction of the dihalodiplatinum(III) complex produces only the lantern diplatinum(II) complex.