The synthesis of derivatives of the tumor-inhibiting metal complex cisplatin (INN), cis-diamminedichloroplatinum(II), is presently aimed at achieving an improved water-solubility, a slower decomposition rate of the substance to toxic byproducts through shielding of the platinum using bulky groups as done in carboplatin,(1) and an improved membrane permeability through an increased lipophilia of the ligands. High lipophilic and kinetically inert Pt(IV) compounds are good candidates for oral application.(2) We used crown ethers, which, owing to their structure and their good solubility properties both in lipophilic and in hydrophilic media, meet the above mentioned requirements and are therefore suitable constituents for new complexes. Several platinum crown ether complexes are known in the Literature,(3,4) but in these adducts the platinum fragment is only Linked through hydrogen bonds to the macrocycle. We previously reported on a four-coordinate crown ether complex in which the platinum is involved in covalent bonding to the crown,(5) which shows reasonable antitumor activity. Here we describe the synthesis, magnetic properties, and X-ray structure analysis of the first example of a five-coordinate platinum crown ether complex with ethylene as the fifth ligand. The structural properties of this compound are in good agreement with those generally made for the rising number of five-coordinate platinum complexes with olefins and other sterically hindered ligands.(6)