Since the discovery of fullerenes and the subsequent incorporation of metal atoms within their carbon cage in 1985, intense interest has focused on these unique molecules. Research has greatly extended our knowledge of the chemical and biological properties of empty fullerenes, but comparatively little is yet known about endohedral metallofullerenes because of their low production yields (102-103 times lower than for empty fullerenes), low solution solubilities, difficult purification, and air sensitivity.
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Lon J. Wilson (1999) studied this question.