Hypoelectronic clusters

The individual structural features of the high-tem-perature superconductors are found in other substances. A substance with alternating metal-salt layers is Ag2F, with sequence FAgAgFAgAgF . Resonance between a covalent bond and a no-bond is found in B (l =3,Z. 6) and in metals and organometallic clusters. Hyperelectronic-hypoelectronic electron transfer occurs... 

KT1 does not have the NaTl structure because the K+ ions are too large to fit into the interstices of the diamond-like Tl- framework. It is a cluster compound K6T16 with distorted octahedral Tig- ions. A Tig- ion could be formulated as an electron precise octahedral cluster, with 24 skeleton electrons and four 2c2e bonds per octahedron vertex. The thallium atoms then would have no lone electron pairs, the outside of the octahedron would have nearly no valence electron density, and there would be no reason for the distortion of the octahedron. Taken as a closo cluster with one lone electron pair per T1 atom, it should have two more electrons. If we assume bonding as in the B6Hg- ion (Fig. 13.11), but occupy the t2g orbitals with only four instead of six electrons, we can understand the observed compression of the octahedra as a Jahn-Teller distortion. Clusters of this kind, that have less electrons than expected according to the Wade rules, are known with gallium, indium and thallium. They are called hypoelectronic clusters their skeleton electron numbers often are 2n or 2n — 4. 

Apparently Hypoelectronic Deltahedra in Bare Clusters of Indium and Thallium Polyhedra with Flattened Vertices... 

Mingos rules. This hypoelectronicity or electron poverty (fewer than the Wade-Mingos 2n + 2 skeletal electrons) in the bare metal cluster anions Enz (z < n + 2) leads to deltahedra not only different from those in the deltahedral boranes but also different from those in hypoelectronic metal carbonyl clusters of metals such as osmium. 

The shapes of the deltahedra in the apparently hypoelectronic clusters of the heavier group 13 metals bear an interesting relationship to their electron counts. All such hypoelectronic deltahedra can be derived formally from the standard closo-borane structures (Figure 1-2) by flattening one or more vertices towards... 

Fig. 1-9. Flattening one to three vertices in 9-, 10-, and 11-vertex deltahedra to give the deltahedra found in apparently hypoelectronic group 13 metal clusters. Vertices of degrees 3, 4, and 6 are indicated by , , and respectively. Vertices of degree 5 are unmarked.

One of the most interesting bare Group 13 metal clusters is the first one to be discovered [66], namely the 11-atom cluster frin in the intermetallic Kglnn-Analogous 11-vertex clusters were subsequently synthesized containing gallium [67] and thallium [68]. The frin cluster has 11+7=18 skeletal electrons = 2 n - 4 for n = 11. It is thus a highly hypoelectronic system relative to the 2n + 2 skeletal electron deltahedral boranes B H . The polyhedron found in Inn is... 

Hypoelectronic clusters have also been identified in solid phases. Hypoelectronic clusters have electron counts less... 

Electron-Poor (Hypoelectronic) Metal Carbonyl Clusters... 

The availability of d orbitals on transition metal vertices leads to the possibility ofelectron-poor or hypoelectronic transition metal clusters with n vertices having less than 2 -I- 2 apparent, skeletal electrons. Such electron-poor clusters form deltahedra containing tetrahedral chambers, i.e., deltahedra with one or more vertices of degree 3 (Figure 9). The simplest examples of such deltahedra are the capped tetrahedra, of which the trigonal bipyramid (i.e., the monocapped tetrahedron) with five vertices is the smallest. The capped tetrahedra consist of a series of fused tetrahedral chambers with faces in common. An example of a cluster based on a bicapped tetrahedron is... 

The extreme hypoelectronicity of the indium and thallium clusters can be relieved by d-orbital participation from some of the vertex metal atoms. In the case of a normal bare post-transition metal vertex in a metal cluster sucb as those discussed in Section 10.6.1, tbe 12 external electrons may be divided into two types, namely, tbe 10 nonbonding d electrons and tbe 2 electrons of an external lone pair analogous to tbe B-H bonding pair in the polyhedral boranes B H (6 < n < 12). In this way a normal post-transition metal vertex such as indium may be considered to use a four-orbital sp bonding manifold just like light vertex atoms such as boron or carbon. However,... 

Cp M firagment withdraws electrons from the skeletal bonding system effectively collapsing the normal closo structure into a highly capped closed cluster core of lower vertex number. These hypoelectronic metallaboranes constitute a new stmcture type in metallaborane chemistry and promise an interesting focus for future research. 

Face capping has been observed only for the rarer chlorides and bromides. The iodide ligand appears to be too large to cap faces, a phenomenon that is also known from early transition metal cluster complexes. For a rare example, there is Nb6 In = Nb6 I gF " 6/2 (Simon et al., 1967), a hypoelectronic 6-8-type cluster with only 19 CBEs. The electron count may be enhanced by encapsulating an H atom (20 CBEs Simon, 1967) and adding a cesium... 

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