The geometries of polynuclear clusters can range in complexity. In bi- and polynuclear compounds, hydrides usually are bridging ligands . Phagocytosis can sometimes continue for more than 20 polynuclear neutrophil nuclei. Many bi- and polynuclear complexes are known with bridging nitrido ligands. Liverpool and Manchester are sometimes considered as one large polynuclear metropolitan area, or megalopolis. Polynuclear metal carbonyls are generally found in late transition metals with low formal oxidation states. Polynuclear anionic species also exist, such as the dark brown , and the black . Characteristically, transition metal alkoxides are polynuclear, that is they contain more than one metal. At higher concentrations, predominantly polynuclear species are formed, including Sn(OH) and Sn(OH). The Grand Calumet suffers from contamination from polychlorinated biphenyls (PCBs), polynuclear aromatic hydrocarbons (PAHs).