Clusters, metal encapsulated atoms

There is much interest in transition-metal carbonyl clusters containing interstitial (or semi-interstitial) atoms in view of the fact that insertion of the encapsulated atom inside the metallic cage increases the number of valence electrons but leaves the molecular geometry essentially unperturbed. The clusters are generally anionic, and the most common interstitial heteroatoms are carbon, nitrogen, and phosphorus. Some representative examples are displayed in Fig. 19.4.3. 

With Ceo, as well as the larger analogs, atoms can be introduced into the internal cavities to form main-group versions of transition-metal clusters containing interstitial atoms. Entities such as main-group atoms like N or a rare gas, molecules tike H2, rare-earth metals and others can be encapsulated. As with external metals, the maximum conductivity occurs for internal metals which are able to transfer three electrons to the radial tiu band of solid C6o-... 

Rhodium also forms an unusually large number of clusters containing encapsulated hetero atoms as, for example, the [Rhn(S)2(CO)32]3 species whose skeletal structure is shown in Fig. 16-15, and a number that contain carbon atoms. It also forms both smaller, for example, Rh CO), and larger, for example, Rh H CO) clusters that have no encapsulated atoms. The Rh12 species has a very novel structure in which two Rl octahedra are linked together to form an Rh6 octahedron between them. Finally, rhodium also forms many mixed metal clusters, especially with plati-... 

As clarified later, such a classification is reasonable and useful because electron transfers exist from the encaged metallic species to the fullerene cages so that the structures and properties depend strongly on the encapsulated atom(s). Particularly, cluster metallofullerenes show different properties from those containing only metals (mono-metallofullerenes and di-metallofullerenes), which, in return, strongly affects the synthesis and extraction processes, structures, chemical reactivities, and their applications. Consequently, we must, to a certain degree, address cluster metallofullerenes separately in the following text. 

For many clusters, the number of valence electrons may be increased as a result of formation of encapsulated compounds containing H, C, N, P, Sb, and S in the interstices between metal atoms (Table 3.12). Recently electron poor clusters containing encapsulated metal atoms such as K and Be and atoms of other elements of the second... 

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°... 

In the field of carbonyl clusters it is not rare to find compounds in which atoms such as C, N and P become trapped in interstitial or semi-interstitial positions inside the metal cage. We will briefly consider this type of compound in order of increasing encapsulation of the interstitial atom. 

Very few examples are known, apart from solid state structures, where the sulfide acts as a bridge to more than four metal atoms. Two Rh clusters [Rh17(S)2(CO)33]3 and [Rh,0S(CO)22]2 have been reported in the literature77,78 where the sulfide is found interstitially in the center of the metal cluster and has contacts to nine and eight Rh atoms, respectively. In the former example an almost linear S—Rh—S unit (with d(Rh—S) as short as 2.16 A) is encapsulated in an Rh16 cluster, with four Rh—S contacts at about 2.33 A and four more at about 2.8 A.77 Other related systems are, for example, the [M6S,7]4 ions (13 M = Nb, Ta) which have been recently prepared by Holm and co-workers.80 Here, among other types of sulfide coordination, a /vS has been found in the base of the bell-shaped ions. 

---