New mercury cyclopentadienyl complexes Hg(eta(1)-Cp')Cl have been prepared by the reaction of HgCl2 and the appropriate KCp' salts or by the transmetalation of HgCl2 with ZnCp'(2) (Cp' = C5Me4H, 1; C5Me4But, 2; C5Me4SiMe3, 3; C5H4SiMe3, 4). By contrast, only the SiMe3-substituted bis(cyclopentadienyl) derivatives, Hg(C5Me4SiMe3)(2) (5) and Hg(C5H4SiMe3)(2) (6), can be isolated by the above synthetic procedures and the appropriate ratio of reagents or from HgCp'Cl and KCp'. Solution NMR studies reveal nonfluxional behavior of the SiMe3-substituted complexes 3, 5, and 6. X-ray studies of the solid-state structures show that the six compounds contain eta(1)-Cp' ligands, with linear or almost linear C-Hg-Cl or C-Hg-C coordination environments. The two HgCp'(2) compounds, 5 and 6, have the expected insular structures, but the HgCp'Cl derivatives show supramolecular associations by means of weak secondary Hg center dot center dot center dot Cl interactions. Thus, the HgCp'Cl compounds 1, 3, and 4 form three different polymeric chain structures with typically two Hg center dot center dot center dot Cl interactions of 3.04-3.46 A per mercury. By contrast, 2 forms a tetramer, [Hg(C5Me4SiMe3)Cl](4), with a cubelike arrangement of four Hg and four Cl atoms. Density functional theory has been used to investigate the electronic structure of the compounds.