Endohedral atom

Crystal structures of a few fullerenes containing endohedral atoms have been recently solved (apart from the noble gas trapping Kr C60,22a the metal atoms containing Sc3 N C78,22b Sc3 N C80 and Sc2 ErN C80>22c Y C82,22d Sc C82,22e Er2 C82,22f Sc3 Cs2,228 Sc2 C84,22h Sc2 C2 C84,221), but unfortunately their electrochemical behaviour is not known. Electrochemical information is also limited for those endohedral complexes which lack definitive structural characterization.23... 

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. 

Such findings indicate that even at room temperature a system with two metastable levels exists, and under certain conditions (for example, under the thermal fluctuations or at the expense of a laser pulse), the endohedral atom can migrate from one stable position in the molecular cage to another. This process is similar to the known effect of caging (easy migration) of the impurity atom in the lattice of heavier atoms however in the case of the studied system all atoms are of the one type. 

It was a great thrust for reduced rare earth metal chemistry when it was discovered that sealed metal ampoules (mostly produced from niobium and tantalum tubing by He arc welding) were reaction containers inert enough to the corrosive molten salts to allow for pure products. Still, all constituents of air need to be excluded, which is possible, thanks to modem dry box techniques. The unintended incorporation of ubiquitous hydrogen or carbon led to a whole new world of cluster complexes with endohedral atoms, mostly with extended stmctmes, as in H Lu Cl or in CLu2 Cl2 (see Rare Earth Metal Cluster Complexes). 

Halides with Cluster Complexes Centered by Endohedral Atoms 419... 

The rare earth elements (Sc, Y, and the lanthanides La-Lu) have only three VEs available for bonding in clusters, ns (n - l)d (n - 2)f f electrons, if present, do not engage in bonding. Therefore, clusters without an endohedral atom ( interstitial ) are extremely rare, although Gd2Cl3 with empty Gdg clusters was the first known example containing a... 

HALIDES WITH CLUSTER COMPLEXES CENTERED BY ENDOHEDRAL ATOMS... 

Figure 10 Isolated cluster complex with endohedral atom,...

Figure 12 Endohedral atoms Z (colored for their different coordination numbers) as found in cluster complexes [ ZRgjX nX g] ...

In an alternative, closed tetramer that was first observed for Ri Ymlho, the endohedral atoms form a tetrahedron. The parent unit has the composition Z4R]6 X36. There is a remarkable structural diversity with compositions as shown in Table 2. Especially remarkable is the composition ZR5X7, for example Ru4Hoi6 l2o nHo4 lg (Figure 23), as it not only contains the iodide-bridged tetramer Ru4Hoi6 l36 but also an empty tetrahedral cluster Ho4 Ig... 

Bonding interactions in these cluster complexes are—as will be pointed out more clearly in Section 5—predominantly of the heteroatomic Z—R and R—X type with only little to no homoatomic Z—Z (only in polymeric stmc-tures possible), R—R, and X X contributions. Thus, the picture usually shown highlighting the octahedral cluster with an endohedral atom in the center ought to be replaced by a picture emphasizing the polar covalent Z—R interactions (see Fig. 5). 

Rare-earth metals R that may form clusters and encapsulate endohedral atoms (or small atom groups such as ( 2)" ) are such that they have 3d (Sc), 4d (Y), or 5d (La, Ce-Lu) states energetically available for Z—R bonding interactions. For the lanthanides, a configuration crossover from a [Xe]4f"5d°6s to a [Xe] 4f 5d 6s must be energetically favorable, which is the case with elements with low third ionization potentials, (see Fig. 6). One may translate this behavior into standard electrode potentials for the half-cells R /R ", E°... 

FIGURE 8 Endohedral atoms Z (colored) as observed in cluster complexes ZR jX,. Numbers and colors (or different shades of grey) indicate coordination numbers of endohedral atcans. 

Structure Before [...] 0 (isolated), 2x0 (dimer), 3x0 (trimer), 4x0 (tetramer), 5x0 (pentamer), 1 (chain), 2 x 1 (double chain), 2 (layer). Inside [...] O (octahedron), TP (trigonal prism), CTP (capped trigonal prism), BCHAP (bicapped hexagonal antiprism), C (cube), SAP (square antiprism), T (transition metal as endohedral atom), 6, 7, 8 (number of cluster atoms), 8, 12, X (number of ligands). After [...] C (cubic), T (tetragonal), O (orthorhombic), M (monoclinic)... 

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