Nido-clusters

Nido Clusters 2n + 4 Systems). Many closo boranes and heteroboranes add two electrons and undergo a concomitant stmctural transformation from a deltahedron to a deltahedral fragment. For instance, closo-2 ()-(Z, [17764-89-OJ, (2n + 2 = 24e ), is readily reduced to... 

In most cases the prolonged treatment of a closo carborane or borane with strong base results in the removal of a single-boron vertex to yield a nido cluster, inert to further degradation. This principle is exploited in the polyhedral contraction and subrogation synthetic strategies. In the prototypical case. 

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. 

Insertion and rearrangement steps were also observed in the reaction of 1 with B2C1443 (Scheme 14). Compounds 52 and 53 were characterized by NMR spectroscopic or mass-spectrometric data, compound 54 by an X-ray crystal structure analysis. 54 may be regarded as a borane-stabilized boranediyl, consisting of a Me5C5B nido-cluster unit (Fig. 8). 

Similarly, the classical tetrahedrane D with six 2c2e bonds may be classified as a nido-cluster having 2n + 4 SE. It is derived from the doso-structure (trigonal-bipyramid), with one apical vertex missing. 

In comparison with the enormous number of carboranes the family of the heavier group 14 heteroborane clusters is yet very small. Most of the heteropoly-borane skeletons with Si, Ge, Sn or Pb atoms belong to the group of larger sized closo- and nido-clusters and the chemistry of these heteroboranes is essentially concentrated on the heteroatoms. 

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. 

The mixed cluster Co4(CO)io(C2Et2) (50) has already been quoted (Fig. 15) as an example of a closo cluster with 6 skeletal atoms and 7 skeletal bond pairs. Nido clusters also formally based on an octahed-... 

The remaining structure illustrated in Fig. 23 is that of the 4-atom nido cluster Fe2(CO)6S2, with 2 apical iron atoms and 2 equatorial sulfur atoms arranged about an incomplete trigonal bipyramid 206). Similar structures are adopted by various diaza complexes of general formula Fe2(CO)6(RNNR) 109). 

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). 

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. 

Scheme 1. Formation of a n complex or a closo or nido cluster.

---