Bis(cyclo-octa-1,5-diene)platinum reacts with benzo-1,4-quinone, 2,3,5,6-tetramethyl-, 2,6-dimethyl-, 2,6-di-t-butyl-, 2,5-dimethyl-, and 2,5-diphenyl-benzo-1,4-quinone to give complexes [Pt(quinone)(cod)](cod = cyclo-octa-1,5-diene). Carbon-13 n.m.r. studies on these compounds failed to establish whether the quinone ligands were η4 or η2 bonded to platinum, and so X-ray diffraction studies have been carried out on the 2,3,5,6-tetramethyl- and 2,6-di-t-butylbenzo-1,4-quinone derivatives. Although crystals of the former exhibited some disorder, preventing complete solution of the structure, it could be deduced that the bonding mode was η4. Crystals of the 2,6-di-t-butylbenzo-1,4-quinone complex were monoclinic, space group P21/n, Z= 4, in a unit cell with lattice parameters a= 10.652(5), b= 12.541(4), c= 14.971(4)Å, and β= 93.17(3)°. The structure has been refined to R 0.025 (R′ 0.027) for 4 258 independent reflections collected at 200 K to 2.9 2θ 60°(Mo-KαX-radiation). The results establish that the 2,6-di-t-butylbenzo-1,4-quinone ligand is attached to the metal by both CC bonds so that with the cod group the platinum atom is in a distorted tetrahedral environment (dihedral angle between the pairs of double bonds of the ligands is 69°). In the complexes [Pt(quinone)(cod)](quinone = 2-methylbenzo-1,4-quinone or naphtho-1,4-quinone or naphtho-1,4-quinone), 13C n.m.r. studies reveal an η2-quinone attachment. Reactions between [Pt(cod)2] and 2,3-dichloronaphtho-1,4-quinone and phenanthrene-9,10-quinone are also described.