Fullerene Ligands
A fullerene ligand is a type of ligand encountered in organometallic chemistry with a fullerene coordinated to a metal. Fullerenes are mostly all carbon, spherical molecules, with the most basic being C60. One of the many uses for fullerenes is their ability to be used as ligands in organometallic systems. Fullerenes were first synthesized in 1985 by Harry Kroto, Richard Smalley, et al. The first use of fullerenes as ligands however did not start until 1991, where C60 is used as a ligand on platinum in the system [(Ph3)P]2Pt(η2-C60). Since this point, there have been many different systems using fullerenes, involving different transition metals and binding modes. Most of the fullerene ligands are based on C60, though there are systems which use different sized fullerenes, such as C70 as seen with C70Rh(H)(CO)(PPh3)2.
Most commonly fullerene ligands behave similarly to electron-deficient alkenes such as tetracyanoethylene. Thus, they coordinate in a dihapto fashion. This binding occurs on the junction points of two 6-membered rings. Hexahapto and pentahapto bonding is rarely present in these systems. Evidence against this lies in alternating bond lengths of the bonded ring carbons. What instead occurs, is multiple dihapto bonding. This can be observed with the ruthenium complex below:
In this example, each ruthenium is coordinated to a separate pair of carbon centres of a 6-membered ring. This bonding is signified with the prefix a μ3-η2:η2:η2, which indicates that the fullerene bridges three metals, and each metal bonds to one pair of adjacent carbon centres. Higher hapticity is observed complexes of C60Ph5−. In this system, bonding to one of the 5-membered acts like a cyclopentadienyl ligand with multiple substituents.
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