Clusters arachno

Arachno Clusters 2n + 6 Systems). In comparison to the number of known closo and nido boranes and heteroboranes, there are relatively fewer arachno species. Partly because of the lack of a large number of stmctures on which to base empirical rules, arachno stmctures appear to be less predictable than their closo and nido counterparts. For example, there are two isomeric forms of one with the arachno [19465-30-6] framework shown... 

In the reaction with BC13 or BBr3 in toluene as solvent, ionic compounds of type 51 were isolated as the main products together with the arachno-clusters 50 (X = C1, Br) and the compounds Me5C5BX2 as byproducts (Scheme 13). The nido cluster cation in 51 is identical with that in compound 48. 

Williams [1] has given an excellent review on Early Carboranes and Their Structural Legacy and he defines carboranes as follows Carboranes are mixed hydrides of carbon and boron in which atoms of both elements feature in the electron-deficient polyhedral molecular skeleton . According to the electron counting rules [2] for closo- (2n + 2 SE), nido- (2n + 4 SE) and arachno-clusters (2n + 6 SE SE = skeletal electrons, n = number of framework atoms) and the An + 2 n electron Hiickel rule, small compounds with skeletal carbon and boron atoms may have an electron count for carboranes and for aromatics (see Chapters 1.1.2 and 1.1.3). 

Insertion of a third methylene bridge into M leads to hypho P with 20 SE. The tetrameric species [Li(thp)2Li87]4 and Li([12]cr-4)2 4(Li87)4 suggest that one Li+ has to be assigned to each [(MeCH)3BMe]32 arachno cluster. The former is coordinated by two C-B bonds of the neighboring cluster [103]. This increases the total... 

Nido-cluster (a nest) a deltahedral structure with one missing vertex. Arachno-cluster (a spider-web) a deltahedral structure with two missing vertices. Hypho-cluster (a net) a deltahedral structure with three missing vertices. 

Pd Bi o]" + is an example of an arachno cluster in the compound [Pd Bi, ) (BiBr j ). It has 2n + 6 skeleton electrons if one assumes one lone electron pair per Bi atom and a neutral Pd atom. The Bi atoms form a pentagonal antiprism which is the same as an icosahedron with two missing vertices. 

A transition element has 5 additional valence orbitals, the 5d orbitals, and therefore 10 additional electrons are required per atom to fill the valence shell of each metal atom. A closo cluster consisting only of transition metal atoms should have a total of 14/i + 2 valence electrons. A capped cluster should have 14n, a nido cluster 14/i + 4, and an arachno cluster 14n+6. The combined formula 4/i+2 + 10m would represent the total electron count for a closo cluster, A mMm, of n atoms that contains m transition metal atoms and n -m main group atoms.Table 8.2 summarizes the main rules, and the following examples show how the total electron counting scheme is applied. 

The S-capped cluster, Cp2Mo2FeS(CO)7 (182), has been prepared from a metal-exchange reaction involving the replacement of two Co(CO)3 groups of FeCo2( i3-S)(CO)9 (183) with two CpMo(CO)2 units.123 The S-or Se-arachno clusters, Cp2Mo2FeS2(CO)7 or Cp2Mo2FeSe2(CO)7, have not been observed. 

Incomplete spherical clusters that require the addition of two fragments to be closed are called arachno-clusters (spider web-like). As the shape is based on a deltahedron of order n but with only n —2 vertices occupied, the number of cluster bonding electrons per cluster fragment is larger than that for closo- and mdo-clusters. It follows that a continuation of the closo-, nido-, arachno- progression eventually leads to electron precise rings or chains. 

Ru6(CO)i8]2-. The same process can be used to generate nido- and arachno-cluster counts, e.g., sep = 7 for both five-atom square pyramid and four-atom butterfly ... 

Exercise 5.3. Consider the observed geometric structure of Cp 2Re2B7Hnwith a cross-cluster Re-Re single bond and a formal sep count of nine. Derive its geometry from nine-sep Cp 2Re2B9H9 by a debor process in order to see if it is an arachno-cluster and a member of the set of open clusters that can be derived from the homologous series shown in Figure 5.18. 

The removal of one or more vertices from a closo polyhedral system leaves the number of skeletal bonding MOs unchanged thus, a 6-vertex closo cluster (octahedral, as in Fig. 5-5), a 5-vertex nido cluster (square pyramidal), and a 4-vertex arachno cluster (butterfly or square planar, depending on whether adjacent or nonadjacent vertices are removed from the octahedron) all have 6 bonding MOs and hence require 7 bonding electron pairs. Expressed in terms of the number of vertices n, this equals n+1, n+2, n+3, and n+4 pairs for closo, nido, arachno, and hypho clusters, respectively. 

Let us now ask how we could predict the correct total electron count, as just defined, for a stable cluster of known structure (i.e., closo, nido, or arachno). To do this for metal carbonyl clusters, it is postulated that in addition to the electrons necessary for skeletal bonding each metal atom will also have 12 nonskeletal electrons. The basis for this assumption is that in the pyramidal M(CO)3 unit each M—CO bond will comprise two formally carbon tr electrons that are donated to the metal atom and two formally metal it electrons that backbond, at least partially, to the CO ligand. Thus, in predicting the total electron count for a closo polyhedral cluster of n vertices, the result would be 12n + 2 n + 1). Similarly, for nido and arachno clusters that are derived from an n-vertex polyhedron (their parent polyhedron) by removal of one or two vertices, respectively, there will be 12 and 24 fewer total electrons, respectively. 

An arachno cluster has a cage based on an n vertex deltahedron with two vertices vacant and has (n + ) pairs of cluster-bonding electrons. 

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