Platinum(IV) metallacrown ethers [PtBr2Me2{im(CH2CH2O)xCH2CH2im}] (im = imidazol-1-yl; x = 2−5, 7; 3−7) and [PtBr2Me2{bim(CH2CH2O)xCH2CH2bim}] (bim = benzimidazol-1-yl; x = 1, 2, 5, 7; 9−12) were synthesized via the reaction of [(PtBr2Me2)n] with the appropriate α,ω-bis(imidazol-1-yl) or α,ω-bis(benzimidazol-1-yl) polyether. Reactions with 1,2-bis(imidazol-1-yl)ethane, bis(2-(imidazol-1-yl)ethyl) ether, or 1,2-bis(benzimidazol-1-yl)ethane yielded the dinuclear complexes [(PtBr2Me2)2{μ-im(CH2CH2O)xCH2CH2im}2] (x = 0, 1; 1, 2) and [(PtBr2Me2)2{μ-bimCH2CH2bim}2] (8), respectively. In addition, the diiodo complex [PtI2Me2{im(CH2CH2O)2CH2CH2im}] (13) was prepared from the reaction of [PtMe2(cod)] with I2 in the presence of im(CH2CH2O)2CH2CH2im. Characterization by microanalysis and NMR spectroscopy, as well as X-ray crystal structure analysis of several of the metallacrown ethers (3, 5, 9 and 10) and dinuclear complexes (2 and 8), is described. The ability of the larger metallacrown ethers (5−7, 11 and 12) to act as hosts to either dibenzyl- or di-n-butylammonium ions is investigated in solution (NMR and ESI-MS). Finally, several of these metallacrown ethers were found to possess in vitro antitumor activity on three tumor cell lines (liposarcoma, A549 and 518A2). The activity of these complexes, all of which were found to induce apoptotic cell death, is discussed relative to their structure and the findings of platinum uptake studies.