Clusters boranes

For c/rwo-boranes and for the larger open-cluster boranes it becomes increasingly difficult to write a simple satisfactory localized orbital structure, and a full MO treatment is required. Intermediate cases, such a.s B5H9, require several resonance hybrids in the localized orbital... 

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... 

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 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. 

Sodium borobydride reacts with Lewis acids in nonprotic solvents to yield diborane [19287-45-7] 2 6 which can then be used to generate other useful organoboranes such as amine boranes, alkyl boranes, and boron hydride clusters. 

The reaction between a trinuclear metal carbonyl cluster and trimetbyl amine borane has been investigated (41) and here the cluster anion functions as a Lewis base toward the boron atom, forming a B—O covalent bond (see Carbonyls). Molecular orbital calculations, supported by stmctural characterization, show that coordination of the amine borane causes small changes in the trinuclear framework. 

B5H9 also acts as a weak Brpnsted acid and, from proton competition reactions with other boranes and borane anions, it has been established that acidity increases with increasing size of the borane cluster and that arachno-boranes are more acidic than nido-horancs ... 

Cluster-expansion and cluster-degradation reactions are a feature of many polyhedral borane species. Examples of cluster-expansion are " ... 

Another important concept is the notion of stabilization by means of coordination, A classic example is the. stabilization of the fugitive species cyclobutadiene, C4H4I by coordination to (Fe(CO)3l (p. 936). As the C atom is isoclectronic with (BH], so (C4H4] is isoelectronic with the borane fragment (BaH which is similarly stabilized by coordination to (Fe(CO)3l or the isoelectronic (Cofr/ -CsHs)) (see Panel on p. 174), Indeed it is a general feature of metallaboranc chemistry that such clusters are often much more stable than are the parent boranes themselves. 

As a result of the systematic application of coordination-chemistry principles, dozens of previously unsuspected stnicture types have been synthesized in which polyhedral boranes or their anions can be considered to act as ligands which donate electron density to metal centres, thereby forming novel metallaboranc elusters, ". Some 40 metals have been found to act as acceptors in this way (see also p. 178). The ideas have been particularly helpful m emphasizing the close interconnection between several previously separated branches of chemistry, notably boron hydride clu.ster chemistry, metallaboranc and metallacarbaborane chemistry (pp. 189-95). organometallic chemistry and metal-metal cluster chemistry. All are now seen to be parts of a coherent whole. 

B Km- At least five different stmcture types of interconnected borane clusters have been identified they have the following features ... 

Derivatives of the boranes include not only simple substituted compounds in which H has been replaced by halogen, OH, alkyl or aryl groups, etc., but also the much more diverse and numerous class of compounds in which one or more B atom in the cluster is replaced by another main-group element such as C, P or S, or by a wide range of metal atoms or coordinated metal groups. These will be considered in later sections. 

The diagrams also indicate why neutral c/oio-boranes BnHn4.2 are unknown since the 2 anionic charges are effectively located in the low-lying inwardly directed orbital which has no overlap with protons outside the cluster (e.g. above the edges or faces of the Bg oct edron). Replacement of the 6 Ht by 6 further builds up the basic three-dimensional network of hexaborides MB6 (p. 150) just as replacement of the 4 H in CH4 begins to build up the diamond lattice. 

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