The maximum pentagon separation rule provides a guideline for the structures of endohedral metallofullerenes

Abstract

Fullerenes tend to follow the isolated pentagon rule, which requires that each of the 12 pentagons is surrounded only by hexagons. Over the past decade many violations to this rule were reported for endohedral fullerenes. Based on the ionic model M(3)N(6+)@C(2n)(6-) and the orbital energies of the isolated cages, in 2005 we formulated a molecular orbital rule to identify the most suitable hosting cages in endohedral metallofullerenes. Now, we give physical support to the orbital rule, and we propose the maximum pentagon separation rule, which can be applied to either isolated pentagon rule cages or to non-isolated pentagon rule cages with the same number of adjacent pentagon pairs. The maximum pentagon separation rule can be formulated as 'The electron transfer from the internal cluster to the fullerene host preferentially adds electrons to the pentagons; therefore, the most suitable carbon cages are those with the largest separations among the 12 pentagons'.

Bottom-up formation of endohedral mono-metallofullerenes is directed by charge transfer

“…As shown in Fig. 5, the smallest detectable cage(s) formed in abundance under the present conditions is M@C 50 for Group 1 elements (K, Rb, Cs), M@C 42 or M@C 44 for Group 2 elements (Ca, Sr, Ba), M@C 36 for trivalent rare earth metals (Sc, Y, La, Ce, Pr, Nd, Gd, Tb, Dy Ho, Er, Lu), although Sc@C 34 can be observed in very low abundance, M@C 44 or M@C 42 for divalent lanthanides (Sm, Eu, Yb, except Tm), and M@C 50 for Ga and In. These results also suggest that ionic radii may be a possible factor that alters EMF formation.…”

“…Negative charge is preferentially located at pentagons or [5,6] bonds in the caged network of metallofullerenes, and to a greater extent when those pentagons are fused to result in [5,5] bonds and [5,5,5] junctions 42 . Consequently, charge transfer stabilizes highly pyramidalized C atoms induced by pentagon adjacency and cages that do not conform to the isolated pentagon rule (IPR) can become quite favourable systems 43 .…”

Sc2O@Td(19151)‐C76: Hindered Cluster Motion inside a Tetrahedral Carbon Cage Probed by Crystallographic and Computational Studies

“…These cages show the lowestn umber of pyracylenes within the IPR and APP1 subsets,a si su sually found for the most stable andc haracterizedc luster fullerenes. [42,43] As ag eneral trend for the lowest-energy isomers shown in Ta ble 1, the relative stabilities of the OCFs (fourth column) are somewhat smaller than those of the tetraanions (third column), which might indicate thatt he interactions between Sc 2 Oa nd non-IPR cages are more favorable than those with IPR cages. This stabilization, however,i sn ot enought or everse the stabilityt rend predicted by the ionic model, that is, for tetra-anions, as found for other cluster fullerenes.…”

Buckyballs

“…the presence of an energetically low-lying fourfold degenerate LUMO, which enables this fullerene to accept up to six electrons, and the electronic distribution may be represented by the ionic model: (Sc 3 N) 6+ @(C 80 ) 6À [195]. Considering the TNT method, endohedrals of the type M 3 N@I h -C 80 have been prepared for very different metals ( M ¼ Sc, Y, La, Ce, Nd, Gd, Tb, Dy, Ho, Er, etc.)…”