The controlled synthesis of isomeric organoplatinum clamshell dimers [Pt2Me2(μ2-κ3-6-dppd)2]2+, 6-dppd = 1,4-di-2-pyridyl-5,6,7,8,9,10-hexahydrocycloocta[d]pyridazine, is reported. The new complexes are formed selectively by self-assembly from mononuclear precursors, taking advantage of the slow cis–trans isomerization at platinum(II). Thus reaction of endo-[PtClMe(κ2-6-dppd)] with AgOTf gave endo,endo-[Pt2Me2(μ2-κ3-6-dppd)2]2+, while the reaction of [PtMe2(κ2-6-dppd)] with HOTf in solvent S = Me2CO or MeCN gave first a mixture of exo- and endo-[PtMe(S)(κ2-6-dppd)]+ and then, by loss of solvent, a mixture of exo,exo- and endo,endo-[Pt2Me2(μ2-κ3-6-dppd)2]2+. The endo,endo isomer slowly isomerized to the more stable exo,exo isomer in solution. Reaction of PPh3 with endo-[PtClMe(κ2-6-dppd)] gave a mixture of endo- and exo-[PtMe(PPh3)(κ2-6-dppd)]+ but reaction with exo,exo-[Pt2Me2(μ2-κ3-6-dppd)2]2+ gave exo-[PtMe(PPh3)(κ2-6-dppd)]+ selectively, with retention of stereochemistry. The structures of the clamshell dimers and of key precursors are reported and equilibria are studied both experimentally and by DFT calculations.