Interstitial heteroatoms

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. 

Because the radius of an octahedral cavity is 0.414 times the metallic radius of the metal atoms on the surface of the cluster, only the first-row main-group atoms can be incorporated in this site. There are several octahedral carbido- and nitrido-clusters, but examples are also known that incorporate hydrogen and boron. A series of related clusters include [Ru6H(CO)i8] , [Ru6B(CO)i7] , [RueQCO)] ], and [Ru6N(CO)i6] , all of whieh contain a fully encapsulated interstitial heteroatom lodged in the center of the octahedral cavity. 

A direct derivation of the above approach made the synthesis of most of the clusters containing B, In, Si, Ge, Sn, P, As, Sb, S, Se, and lb interstitial heteroatoms or elemental organic fragments possible by the cleavage of E-H, E-G and E-C bonds under suitable experimental conditions. 

The incorporation of interstitial or encapsulated heteroatoms is a common and stabilizing feature. Carbon is the most common and, as is the case in... 

A simplified model may be used to rationalize the bonding in the heteroatomic species [Zn ZnsBi4 Bi7]5. According to the electron counting theory proposed by Wade, the formation of a closo deltahedra of 12 vertices is stabilized by 13 skeletal electron pairs. The total of 26 electrons required for skeletal bonding may be considered to be provided as follows 2 from the interstitial Zn atom, (8x0 + 4x3 = 12) from the ZngBi4 icosahedral unit (each vertex atom carries an exo lone pair or bond pair), 7 from the capping Bi atoms, and 5 from... 

These defects can occur in several classes that are depicted in Figure 4. Point defects may consist of voids (missing atoms) in a lattice. Equally frequently the constituents of the solid or heteroatoms are located at interstitial sites. These defects may be statically distributed giving only indirect evidence of their presence by small deviations of the lattice parameters (a fraction of a percent up to few percent). Frequently,... 

The description of a heteroatom as interstitial is well-defined when it is completely encapsulated within a polyhedron of metal atoms as, for example, in the octahedral cluster [Ru6C(CO)i7].l l There is, however, a large number of clusters containing naked main-group atoms in a variety of coordination modes which cannot be classified as readily. For example, the carbon atom in [Ru5C(CO)i5] lies slightly below the basal plane of a square pyramidal cluster framework where it... 

For tetranuclear clusters to incorporate main-group elements interstitially, they must adopt a more open skeletal arrangement. The M4 butterfly cluster framework can be derived from the tetrahedron by cleaving a metal-metal edge alternatively, it can be viewed as an orac/mo-octahedron (Scheme 1). Butterfly clusters containing semi-interstitial B, C, N, and O atoms have been observed and they all adopt similar structures in which the heteroatom interacts with all four metal atoms and is located in a rather exposed position midway between the wing-tip atoms of the M4 skeleton. 

As for tetranuclear clusters, pentanuclear transition metal clusters are only capable of partially encapsulating a main-group heteroatom. The c/o5o-pentanuclear cluster, the trigonal bipyramid, has an interstitial cavity of similar dimension to that of the tetrahedron, and must therefore open-up if it is to accommodate a semi-interstitial atom. The most commonly observed M5E clusters (E = main-group atom) adopt square based pyramidal structures, which can be derived from the trigonal bipyramid by M-M bond cleavage or can, alternatively, be viewed as /i/Jo-octahedral species (Scheme 2). 

Carbon and nitrogen atoms have been found to occupy such semi-interstitial positions in the clusters [M5C(CO)i5] (M = Fe, Ru, or Os)l i and [M5N(CO)i4] (M = Fe or The heteroatoms in each cluster lie in a slightly exposed... 

Interest in the synthesis of transition metal carbonyl clusters incorporating interstitial main-group atoms is because they provide a conceptual bridge between or-ganometallic chemistry and the areas of inorganic solid-state and surface chemistry. In addition to serving as useful models for either solid-state binary alloys or for the chemisorption of heteroatoms on the step-site of a metal surface, these discrete molecular clusters are often markedly more stable, especially toward the temperatures and pressures required for catalytic reactions. 

Examination of the lattice packing in crystals of compounds with a preponderance of Ph in P gives an impression that these crystals are very similar to those of crystalline aromatic hydrocarbons, except for a small proportion of interstitial atoms from domains M and L. The aromatic hydrocarbon analogy is not useful, however, because the configurations and conformations of the Ph groups are restricted by their connections to the heteroatoms in domain L. 

Many dusters incorporate heteroatoms such as H, C, N, or even As, Sb, P, S, Ge, etc. inside their metallic cores (see Chapter 3.2). [189] The metal core structure usually forms a cavity in which the heteroatom is accomodated. The metallic core can be fundamentally cubic or pentagonal. The number of interstitial atoms embedded within the metal core depends on the geometry and the dimendons of the cluster. In some cases a small molecule is encapsulated in the metal duster, for example, in the dusters [Co6Ni2C2(CO)i4p [190] and [Nii6(C2)2(CO)23] " [191]... 

An important borderline is reached with the last example. Whatever the actual distribution of valence electrons in 1 2820 may be, the formal ionic limit of [(Tk )2(S )2C "(e )2] leaves electrons for (extended) M-M bonding. In contrast, all electrons are used for strong heteroatomic bonds in Gd2Br2C ( [(Gd )2-(Br )2C ]). Here, the stabilization of the clusters by the interstitial atoms has finally led to a destruction of the cluster, at least if M-M bonding is a requirement for using the term cluster . Gd2Br2C is a normal and simple ionic compound just as the isostructural and isoelectronic La2028. [164]... 

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