Carbon dioxide binds to metals in only a few ways. The bonding mode depends on the electrophilicity and basicity of the metal centre. Most common is the η-CO coordination mode as illustrated by Aresta's complex, Ni(CO)(PCy), which was the first reported complex of CO This square-planar compound is a derivative of Ni(II) with a reduced CO ligand. In rare cases, CO binds to metals as a Lewis base through its oxygen centres, but such adducts are weak and mainly of theoretical interest. A variety of multinuclear complexes are also known often involving Lewis basic and Lewis acidic metals, e.g. metallacarboxylate salts (CH)Fe(CO)COK. In multinuclear cases (compounds containing more than one metal), more complicated and more varied coordination geometries are observed. One example is the unsymmetrical compound containing four rhenium centres, [(CO)ReCORe(CO)]. Carbon dioxide can also bind to ligands on a metal complex (vs just the metal), e.g. by converting hydroxy ligands to carbonato ligands.