Arachno structures

Fig. 2.1-23. Rearrangement of nido-C R>, H3 via an arachno-structure in the presence of an ether L.

For a B Hm borane based on an anion B H 6-, an arachno-structure is preferred with B at n vertices of an (n + 2) vertices polyhedron (n + 3) pairs of framework bonding electrons are required. 

The problems outlined in the previous section can be avoided if, instead of allocating the skeletal bonding electron pairs to localized bonds, one simply compares their number with the number of skeletal bonding MO s (199). The closo, nido, and arachno structures of boranes and carboranes can then be seen to reflect the numbers of skeletal bond pairs that are available to hold their skeletal atoms together. 

Compounds with a skeletal atoms and b skeletal bond pairs adopt closo structures if 6 = (o + 1), nido structures if 6 = (a + 2), and arachno structures if 6 = (o + 3). 

Mixed Clusters with Closo, Nido, or Arachno Structures ... 

Fig. 25. The arachno structures of some metal-hydrocarbon w complexes. (Carbonyl ligands not shown.)...

Arachno structures among main group metal-hydrocarbon complexes... 

Fig. 28. (a) The arachno structure of benzvalene and nido structures of C5H5+ and CsMee . (b) The boron heterocycles Ph4C4BPh and H4C4B2F2. 

If the bonding in BgHig and in C2BgHxo is described in terms of 2- and 3-center bonds, then four 3-center bonds should reside within the skeletal framework and, in spite of the more open arachno structure, the required four 3-center bonds are successfully accommodated, at least in the crystalline phase. The possibility that the true nido structures (I-N8 for BgHi2 and X-N8 for CgBgHio) may exist in the fluid phases is under investigation. 

Since a rather unstable C2BaHio ido-carborane had been known for several years to exist, this isomer of C2BeHio was structurally identified (41) as having (probably) the arachno structure of V-N8 instead of the nido structure X-N8, but this is not yet sure. 

Cationic deltahedral Pn clusters are known for Pn=Bi. [Bis], [Big], and [Bi9], which have been structurally characterized with complex counterions such as AlCLt, AsFg, and HfCle , can be obtained either by molten salt routes or by using so-called super acidic systems [205-209]. According to Wade s formalism, [Bis] " " and [Big] correspond to 12 ske closo and 22 ske arachno structures, respectively, and the latter adopts the shape of a square antiprism. [Bi9] " which is expected to form a 22 ske nido-cluster in analogy to Fig. 2d, rather adopts a distorted tricapped trigonal prismatic topology (Fig. 2c). 

According to Wade s rules, one non-bonding electron pair is allocated to each metal atom in polyionic metal clusters of the p-block elements. Thus the planar five-membered M5 anions are 16-electron species comprised of two valence electrons contributed by each metal atom and the 6- charge. Since there are eight electron pairs for bonding five cluster atoms, Wade s rules predict an arachno structure ( + 3 electron pairs for n cluster atoms), i.e. a pentagonal bipyramid with two vacant sites. 

Arachno Clusters (2 n + 6 Systems). In comparison to the number of known closo and nido boranes and heteroboranes, there are rdatively fewer arachno species. Pardy because of the lack of a large number of structures on which to base empirical rules, arachno structures appear to be less predictable than their closo and nido counterparts. For example, there are two isomeric forms of B9H15, one with the arachno [19465-30-6] framework shown in Figure 2 (33), the other with a framework more reminiscent of that shown for the nine-atom nido classification (34). Structures of arachno molecules involve the presence of even more extra hydrogens or other electron-donating heteroatoms than nido molecules. Typical examples are given in Table 1. 

If we remove two vertex boron atoms, the resulting framework is an arachno (Gr., spider s web ) structure. With two vertices missing, the structure is even more open than is the nido case and the resemblance to the parent closo structure is less appurent. Arachno structures obey the electronic formula 2n + 6 (or n + 3 electron pairs). Pentaborane(l I), BjB,. must therefore have an arachno structure. In the arachno series the extra hydrogen atoms form ertdo B—H bonds (lying dose to (he framework) as well as bridges. 

Antiferromagnetism, 467-468 Aprotic solvents, 369-373 Aqueous solvents, and non-aqueous solvents. 359-386 Arachno structures, 798-800, 807 Aragonite, 98, 953 Arene complexes, 681-683 Arrhenius acid-base definition, 319... 

Both 10 and 11 react with halide ions to give [M5C(CO),5X] products. The iodopentaosmium cluster [Os5C(CO)I5I], 12, has been fully characterized by X-ray diffraction (Fig. 12) and adopts an open arachno structure based on a pentagonal bipyramid but lacking two equatorial vertices (31). 

In clusters composed solely of main-group elements, the following electronic relationship between closo, nido, and arachno structures has been found (12, 29) ... 

X-ray studies have established that /9-R105 boron has a very porous (only 36% of space is filled in the idealized model) and defective structure with the presence of interstitial atoms and partial occupancies. The B57 fragment can dispose of excess electrons by removal of some vertices to form nido or arachno structures, and individual Bi2a units can gain electrons by incorporating capping vertices that are accommodated in interstitial holes (see Fig. 13.4.11(b)). 

The structure of B5HU is the classical arachno-structure obtained by the removal of a basal boron atom from B6K10. The molecular structure is that of a shallow square pyramid with an open side. The structure, shown in Fig. 17, which was determined by low temperature X-ray diffraction, is that of an asymmetric molecule with Cj symmetry1151. The terminal hydrogen H2 is actually bridging to one of the basals borons, B2 or B5. The distances, B2—H2 and B5—H2 are clearly asymmetric, being 1.55 A and 1.83 A, respectively. 

The structures of boranes can be grouped into several classifications. If the structure contains a complete polyhedron of boron atoms, it is referred to as a closo borane (closo comes from a Greek word meaning closed ). If the structure has one boron atom missing from a comer of the polyhedron, the structure is referred to as a nido borane (nido comes from a Latin word for nest ). In this type of structure, a polyhedron having n comers has (n - 1) comers that are occupied by boron atoms. A borane in which two comers are unoccupied is referred to as an arachno structure (arachno comes from a Greek word for web ). Other types of boranes have structures that are classified in different ways, but they are less numerous and will not be described. 

The arachno structure of B4H10 (a) and the nido structure of B5H9 (b). 

Figure 3-20. Examples of closo/nido and closo/arachno structural relationships after Muetterties [44], Top Closo-B6H62 and nido-B5H9 Bottom Closo-B7H72 ...

---