Fullerene derivative 1 containing of an α-ylide group exists as a mixture of E and Z isomers. Treatment of 1 with MgBr2 or LiI in CDCl3 under ultrasonic agitation gave an adduct in which the carbonyl oxygen of α-ylide ester moiety in 1 is coordinated to the metal ion. The NMR spectra show that the adducts exist as a single isomeric form. After ultrasonic agitation, the adducts gradually revert to the original E, Z-mixture owing to slow absorption of moisture. Further ultrasonic treatment of the mixture resumes the complexation as long as enough anhydrous metal halides are still present.