The complex cis-[Pt(CO)2Cl2], in chloroform solution at room temperature, reacts with methyl 2-butynoate in the presence of water and carbon monoxide to give [Cl(CO)P[graphic omitted]t(CO)Cl](1). Complex (1) is easily transformed into the related triphenylphosphine derivative [Cl(CO)P[graphic omitted]tCl(PPh3)](2). A multinuclear (1H, 195Pt, and 31P) n.m.r. study of complex (2) is reported. The analysis of the 195Pt n.m.r. spectrum gave unambiguously the value 3J(Pt–Pt)= 818 Hz. The crystal structure has been solved by Patterson and Fourier methods from counter data and refined by blocked full-matrix least squares to a final conventional R of 0.0317 for 3 795 independent observed reflections. Crystals of complex (2) are triclinic, space group P, with unit-cell dimensions a= 13.921(4), b= 10.386(3), c= 10.006(3)Å, α= 107.56(7), β= 103.02(6), γ= 94.36(6)°, and Z= 2. The crystal contains discrete molecules of (2) with square-planar arrangements around the Pt atoms showing large angular distortions due to the two five-membered rings. The two fused metallacycles and the two co-ordination planes are essentially coplanar, giving a flat molecule. No interaction between the two metal centres is present [Pt(1) Pt(2) 4.721(1)Å]. The bond distances between the metal atoms and the organic fragment are significantly different: the Pt–C(sp2) lengths are 1.944(7) and 1.996(9)Å, with a trans chlorine atom in both cases, while the Pt–O distances are 2.118(5) and 2.017(5)Åtrans to the phosphine ligand and to the terminally bonded carbon monoxide, respectively.