Syntheses and reactions of the cis-PtCl2{Ph2P(CH2CH2O)nCH2CH2PPh2-P,P'} (n = 3-5) metallacrown ether complexes. The x-ray crystal structures of the n = 4 and 5 complexes and of [cis-Pt{Ph2P(CH2CH2O)4CH2CH2PPh2-P,P',O}(H2O)](BF4)2