Icosahedral clusters stability

I contrast to cages like B12X12 or Cgg, clusters with similar sizes consisting of metal atoms are not stable if they are hollow the bonds at their surfaces are too weak. However, Urey can be stabilized by interstitial atoms, even if the interstitial atoms do not contribute with their electrons. Such clusters are called endohedral. Examples are the icosahedral clusters [Pt Pbi2] and [Cd Tli2] 2 with a Tl cage. The atom mentioned before the sign is the enclosed, endohedral atom. These clusters fulfill the Wade rule for closo clusters if one assumes a neutral Pt atom and a Cd ion. 

The deltahedron for n = 10, a bicapped square antiprism, exhibits two four-connect and eight five-connect vertices. Hence, for one heteroatom in a ten vertex c/oso-cluster we have 1 - and 2-isomers and two heteroatoms in 1,10-, 1,6-1,2-, 2,3-, 2,4-, 2,6- 2,8-isomers. Different placements generate different cluster stabilities. A rule of thumb is that the more electronegative element prefers the lower-connectivity vertex. Multiple heteroatoms more electronegative than B prefer non-adjacent positions as far apart as possible. Rearrangement to the most stable isomeric form need not be fast. In the case of icosahedral clusters, for example, the barrier to rearrangement is large and isomers can be isolated. 

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