Clusters endohedral atoms

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. 

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)... 

The only isolated cluster complex, with no chloride ligands shared, is known for [ CLu6 Clig]Cs2Lu (Fig. 2), with rle = 6.0 and x/r = 3.0, with a single carbon atom as the endohedral atom (Artelt et al., 1992). 

FIGURE 10 Endohedral atoms observed for ZR6IX12R- and (ZR lXio-type cluster complexes. Red (dark grey) elements occur as endohedral atoms in both structure types, and orange (hght grey) elements only in the 7-12 type. 

Cluster condensation begins with two clusters sharing one or more atoms. At present, oligomers with endohedral transition metal atoms are only known with octahedral clusters, TRs. Obviously, T atoms are too big to be accommodated in tetrahedra or trigonal bipyramids. On the other hand, oligomers always compete both thermodynamically and electronically with chains the more cluster atoms are shared, the lesser the electron cotuit has to be, scaled to one endohedral atom. 

---