The \(\pi\) systems of fullerenes (eg buckmisterfullerene, \(\ce{C60}\)) also act as ligands for metal complexes. Most transition metal-fullerene complexes are derived from \(C_{60}\). The structure of \(\ce{C60[IrCl(CO)(PMe3)2]2}\) is shown in Figure \(\PageIndex{1}\). As ligands, fullerenes behave similarly to electron-deficient alkenes, and they prefer coordination to electron-rich metal centers (metal ions with softer character like those of the 4d and 5d transition metals). They almost always coordinate in a dihapto (\(\eta^2\)) fashion. The \(\eta^2\) metal binding most often occurs on the junction of two 6-membered rings, as shown for \(\ce{[[Ph3P]2Pt]6(\eta^2-C60)}\) on the left of Figure \(\PageIndex{2}\). In (\ce{Ru3(CO)9(C60)}\), the fullerene binds to the triangular face of the cluster as shown on the right in Figure \(\PageIndex{2}\). Hexahapto and pentahato binding is possible, but is less common that dihapto coordination. Modification of the fullerene with phynyl substituents makes the fullerene a more electron rich ligand so that penta- and hexahapto coordination is more favorable. For example, the pentaphenyl anion, \(\ce{C60Ph5^-}\), binds to Fe in a pentahpto (\(\eta^5\)) fashion, similar to the interactions in ferrocene (Left, Figure \(\PageIndex{3}\)). The first X-ray structure that gave insight into the spherical structure of fullerenes was derived from an oxygen adduct of osmium tetroxide (Right, Figure \(\PageIndex{3}\)).