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Endohedral metallofullerenes have attracted special interest as spherical molecules with novel properties that are not expected from empty fullerenes.['] The higher fullerenes can even encapsulate two metal atoms inside the carbon cages to form soluble and relatively air-stable endohedral dimetallofullerenes. Because of the difficulty in producing pure samples in large quantities, experimental characterization of these species has been hindered. Important progress was marked by the successful isolation and purification of endohedral dimetallofullerenes such as Sc2 (iC,,12] and La,@C,,[31 in macroscopic quantities. This has made it possible to investigate redox propertiesr3] and reactivity.14] STM''] and TEMI61 studies were also carried out in an attempt to confirm that two metal atoms are inside the fullerene cages.Since there are many isomers for higher fullerenes, determination of cage structures and symmetries has long been of fundamental interest in disclosing the mechanism of growth. This has been a recent subject of theoretical14. ' * and experimental studies. ['-l o ] Since its first observation in 1991[111 La,@C,, has been widely known as a representative and abundant dimetallofullerene. Despite several attempts, its cage structure and symmetry have not yet been experimentally confirmed. In addition, it is currently of increasing interest whether encapsulated metal atoms are rigidly attached to fullerene cages or move about freely. Although rotation of a molecule within a cage is of great help in designing functional molecular devices,['21 very little has been known about the dynamic behavior of metal atoms encapsulated inside carbon cages. Based on 13C and 13'La NMR spectra of La,@CEO, we now report its structural determination and the first experimental evidence for circular motion of two La atoms.For the C,, fullerene there are seven distinct isomers (D,, D,,, Czv, CZv., D,, D,,, and Zh) that satisfy the isolated-pentagon rule.[131 The isomer distribution has been an open question, since C,, was a "missing" fullerene between C,, and C,, . By isolating C,, and analyzing the observed I3C NMR lines, however, it was verified that the D, isomer is most abundantly produced ( > 90 %) .[I4] It seems plausible to assume that two La atoms are encapsulated inside the D, isomer. However, theoretical calculationsl8I have shown that encapsulation of two La atoms inside the most unstable Z, cage (about 52 kcal mol-' less stable than the D, isomer) is most favorable (63 kcalmol-' more stable than encapsulation inside the D, isomer). This is because the I,,-symmetrical C,, has only two electrons in the fourfold degenerate HOMO and can accommodate six more electrons to form the stable, closed-shell electronic state of (La3'),C:;with a large HOMO-LUMO gap. The most stable endohedral structure optimized with the I, cage is shown in Figure 1. It has D,, symmetry; two La atoms are located equivalently on the C, axis facing the hexagonal rings of C,, with a long La-La distance of 3.655 A.r81 The 139La NMR spectrum of La,@C...