Clusters interstitial main group elements

Hexacapped cubic species with an interstitial main group element instead of a transition metal are rather scarce. Hitherto, only one example, namely the 119-MVE compound Nig(//g-As)(/i4-As)6(PPh3)8 (6) has been prepared. The structural arrangement of this cluster reported by Fenske and coworkers is strongly related to that of the metal-centered Ni9(//4-As)6(P-Bu"3)g with comparable Nis Nis and Nij-As separations. 

A. transition metal clusters containing interstitial main group elements (Table 3-8), [246-250]... 

Geometrically, the main group element tends to retain a tetrahedral nearest neighbor environment, whereas the transition metal element tends to retain an octahedral environment. As a consequence, transition metal clusters with more than six metal atoms have a tendency toward ligand loss, leading to the formation of condensed clusters (multiple interstitial metal atoms). This leads eventually to close-packed structures that mimic bulk metal structures (see Polynuclear Organometallic Cluster Complexes). On the... 

Interstitial carbon atoms are a unique aspect of organometallic cluster chemistry (see Cluster Compounds Inorganometallic Compounds Containing Transition Metal Main Group Element Carbon atoms that are completely... 

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. 

The hexacobalt and hexaosmium cluster anions, [Co6P(CO)i6] and [Os6P(CO)i8] are interesting examples which illustrate the effects of introducing a larger, second-row, main-group element into a six-coordinate interstitial cavity. They also enable a comparison of the differences between the cavity sizes for first and third-row transition metals to be made. 

Mathematical cluster chemistry/Metal-metal interactions in transition metal clusters with donor ligands/Electron count versus structural arrangement in clusters based on a cubic transition metal core with bridging Main Group elements/Metalloboranes/ Clusters with interstitial atoms from the p-block How do Wade s rules handle them /Diverse naked clusters of the heavy Main Group elements Electronic regularities and analogies... 

The most frequently encountered interstitial species are the second period main group elements C and N. From the main group atoms in the periodic table, B and Si have also been reported to occupy interstitial sites in the reduced rare earth halides. Their valence atomic orbitals, s, p,, p and p, transform as a g + ti in an octahedral field (Bursten et al. 1980). In addition to the ajg orbital, the metal-metal bonding levels of the clusters also contain a t u orbital which is well suited to overlap strongly with the orbitals on the interstitial. Figure 41 illustrates the molecular orbital diagram... 

Overview of the Synthetic Methods and Spectroscopic Characteristics of Transition Metal Clusters Containing Interstitial or Exposed Main Group Elements... 

We shall discuss in more detail only the synthesis of the exposed and interstitial carbides, since this class of clusters is one of the most investigated and well developed. Moreover, their syntheses require methods which have been, or can likely be, extended to other main group elements. 

A few of the synthetic methods adopted so far are closely related to the previously examined methods used for the synthesis of clusters containing interstitial or exposed main group elements or elemental organic fragments. For example, as shown in Scheme 3-11, the very peculiar [Bi4Fe4(CO),3] cluster (see Section 3.2.6) has been obtained in two steps through classical redox condensation reactions or by the clockwise condensation induced by fragment elimination. [312] Related syntheses have also been used for the E-Co (E = Sb,Bi) [314, 315] and n-Fe [309] mixed clusters listed in Ihble 3-11. 

S.3 Structural characteristics of TVansition Metal Clusters Containii Interstitial or Exposed Main Group Elements or Elemental Organic Fragments... 

In both metal lattices and closo polyhedral metal clusters there are cavities present, whose dimensions are a function of the number of vertices, the shape of the polyhedral moiety, and the interatomic separation. Partial occupation of these sites within the metal lattice by main group elements results in the formation of interstitial alloys such as, for example, metal hydrides, carbides, and nitrides. Occupation of the cavity within a molecular metal cluster gives rise to interstitial clusters. Although close topological relationships are sometimes found between interstitial alloys and interstitial clusters, the greater degree of freedom of a molecular assembly of metals, in comparison to a three dimensional infinite array of metals, increases the possible number of interstitially lodged elements... 

It is not intended here to give a full account of all the variety of structures which can be stabilized by the incorporation of interstitial atoms or the peripheral coordination of main group elements. However, a few more examples seem appropriate so as to give some indication about the potential vastness of this area of cluster chemistry. 

M5 and M6E are typically encapsulated structures and similarly to M4E species are accessible only when E is a first row element. Larger main group elements are not appropriate for these cavities. Selected examples of clusters with stoichiometries M4E, M5E, and M6E are shown in Table 3.2. The most important family of clusters with interstitial or encapsulated atoms are by far the carbides. 

Typical structural motifs for S-, Sc-, and Te-containing iron cluster compounds shown in Scheme 9 (E, E = S, Se, Te cf. Scheme 1) involve the tetrahedral monocapped species with the closed iron triangle 118 and the dicapped triiron species with the open Fcs framework 121, which can also be regarded as a square-pyramidal c2,EE skeleton with an Fcafifi basal plane 121. The protonated forms of 118 (119 and 120) and the dicapped octahedral structure 122 based on an Fc4 square are also known. Butterfly 11 and interstitial structures (14 and 15 Scheme 1) are very rare for non-first-row main group elements, and a rare example of the /44-S butterfly cluster 116 is discussed in Section... 

Clusters with interstitial atoms are an interesting class of polynuclear complexes. Several examples of compounds with interstitial atoms (H, C, Ge, N, P, S, etc.) have been reported. These compounds exhibit special behavior because the presence of main-group atoms in the cluster cage influences the chemical reactivity and stability of the system and its geometric structure. In the following section we consider some of these peculiarities, related both to the stability and to the electronic properties of clusters with interstitial elements. 

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