The preparation, X-ray crystal structure, and the solution behavior of a cyclic, tetranuclear nucleobase complex of (en)Pt(II), [(en)Pt(UH-N1,N3)]4(NO3)4 (2) (with en = 1.2-diaminoethane, UH = uracil monoanion, N1,N3 = platination sites) is described. The tetranuclear cation of 2 displays close similarities with calix[4]arenes as far as overall geometry and conformational behavior is concerned. The affinity of the metal analogue 2 for metal cations appears to be more pronounced as compared to typical calix[4]arenes, however. In the solid state, 2 adopts a 1,3-alternate conformation of the four uracil rings, but in solution and after deprotonation to [(en)Pt(U-N1,N3)]4 a second major species (cone conformer) occurs in solution. The assignment of this species is made on the basis of preliminary X-ray crystal data of a Ag4 derivative of 2, [(en)Pt(UH)Ag]4(NO3)8.4HO (3), in which the four uracil rings adopt a pinched-cone conformation and which, after Ag+ removal by means of Cl-, produces an identical species. 2 forms spontaneously in aqueous solution from the mononuclear precursor [(en)Pt(UH-N1)(H2O)](NO3).H2O (1). The analogous cis-[(NH3)2Pt(UH-N1)(H2O)](NO3) seems to behave similarly, although condensation products have not been isolated. In contrast, the N(3) linkage isomer cis-[(NH3)2Pt(UH-N3)(H2O)](NO3), prepared in situ from (cis-NH3)2Pt(UH-N3)Cl.2H2O(4) and AgNO3, does not form a cyclic tetramer under comparable experimental conditions but dimerizes only to a head-tail species with N3,O Pt coordination and a mu-OH complex, depending upon pH. Crystallographic data for the three structurally characterized compounds 1, 2, and 4 are as follows : 1, triclinic system, space group P1, a = 7.229(1) angstrom, b = 9.732(3) angstrom, c = 9.968(2) angstrom, a = 107.37(2)degrees, beta = 103.14(1)degrees, gamma = 99.46(2)degrees, Z = 2; 2, tetragonal system, space group P4(2)/n, a = 16.117(8) angstrom, b = 16.131(8) angstrom, c = 8.626(3) angstrom, z = 4; 4, monoclinic system, space group P2(1)/c, a = 6.592(14) angstrom, b = 20.135(25 ) angstrom, c = 7.493(10) angstrom, beta = 92.32(14)degrees, Z = 4. The cyclic tetramer 2 can be considered a prototype of a metal analogue of a calix[4]arene. Implications for the synthesis of other metal analogues of calixarenes and possible host-guest chemistry are briefly pointed out.