Cluster compounds boron hydrides

The favored geometry in coordination compounds, boron hydrides, and metal clusters is the polyhedron that has all faces equilateral or near equilateral triangles... ... 

There are, however, also a few inorganic diazo compounds in which X has 18 or more atoms and, in addition, the members of this group have a stability and reactivity comparable to those of aromatic diazo compounds. These are the mono- and bis(diazo)boranes, which were discovered by the group of Muetterties in their work on cluster-type boron hydrides (Knoth et al., 1964). 

The nomenclature of boron hydride derivatives has been somewhat confusing and many inconsistencies exist in the Hterature. The stmctures of some reported boron hydride clusters are so compHcated that only a stmctural drawing or graph, often accompanied by explanatory text, is used to describe them. Traditional nomenclature systems often can be used to describe compounds unambiguously, but the resulting descriptions may be so long and unwieldy that they are of Htde use. The lUPAC (7) and the Chemical Abstract Service (8) have made recommendations, and nomenclature methods have now been developed that can adequately handle nearly all clusters compounds however, these methods have yet to be widely adopted. Eor the most part, nomenclature used in the original Hterature is retained herein. 

Wade electron counting rules borane-like cluster nomenclature. On initially studying compounds such as boranes (boron hydrides) and carboranes (or carbaboranes boron—carbon hydrides), Wade (1976) proposed a number of rules which have then been extended to several compounds and which relate the number of skeletal electrons with the structure of deltahedral clusters. A polyhedron which has only A-shaped, that is triangular, faces is also called a deltahedron. 

Al, Ga, In and T1 differ sharply from boron. They have greater chemical reactivity at lower temperatures, well-defined cationic chemistry in aqueous solutions they do not form numerous volatile hydrides and cluster compounds as boron. Aluminium readily oxidizes in air, but bulk samples of the metal form a coherent protective oxide film preventing appreciable reaction aluminium dissolves in dilute mineral acids, but it is passivated by concentrated HN03. It reacts with aqueous NaOH, while gallium, indium and thallium dissolve in most acids. 

The boron hydrides, including the polyhedral boranes, heteroboranes, and their metaUa derivatives, encompass an amazingly diverse area of chemistry. This class contains the most extensive array of structurally characterized cluster compounds known. Included here are many novel clusters possessing idealized molecular geometries ranging over every point group symmetry from identity (C[) to icosahedral (I[). Because boron hydride clusters may be considered in some respects to be progenitorial models of metal clusters, their development has provided a framework for the development of cluster chemistry in... 

Another complex hydride, A1(BH4)3 (9), a colorless liqnid (mp -64.5 °C, bp 44.6 °C), was the first componnd demonstrated to be flnxional (see Fluxional Molecule). Its thermal decomposition also led to the first componnd to be discovered and structurally characterized by NMR spectroscopy, AI2B4H18 (10). Reaction of A1(BH4)3 with a variety of boranes produces compounds analogous to boron hydride clusters such as AIB4H u, AIB5 H11, AIB5 H12, A1B6Hi2, and AlBeHn (seeBoron Hydrides) ... 

Metallacarbaborane (also called metallacarborane) an electron-deficient compound, usually a polyhedral cluster comprising an array of boron-hydride (BH), carbon-hydride (CH), and metal (ML, where L = ligand) fragments the inclusion of a substituted carbon fragment CR (R = alkyl, aryl, or trimethylsilyl) in place of a CH unit is common... 

Boron Hydrides Boron Inorganic Chemistry Boron Organoboranes Boron Polyhedral Carboranes Cluster Compounds Inorganometalhc Compounds Containing Transition Metal Main Group Elements Electronic Structure of Main-group Compounds. 

Hydrogen bridges between the beryllium atoms produce a polymeric structure for BeH2, as shown in Fig. 18.6. The localized electron model describes this bonding by assuming that only one electron pair is available to bind each Be—H—Be cluster. This is called a three-center bond, since one electron pair is shared among three atoms. Three-center bonds have also been postulated to explain the bonding in other electron-deficient compounds (compounds where there are fewer electron pairs than bonds), such as the boron hydrides (see Section 18.5). 

The TEC model developed by Teo also has been successfully applied to rationalize the geometries of a large number of cluster compounds. The TEC model combines Lauher s rule with Euler s theorem and adds an adjustable parameter This parameter X is equal to the number of electron pairs present in excess of that predicted by the 18-electron rule. " X has also been interpreted in terms of the number of missing antibonding orbitals. Given a value for X, determined by the shape of the cluster, an equation predicts the electron count for a cluster. Theoretical justification of the parameter X is based largely upon the classical molecular orbital calculations performed by Hoffmann and Lipscomb via the extended Hiickel method on the corresponding polyhedral boron hydride clusters The values... 

Figure 1.3 shows the structures of some deltahedral (i.e., triangular-faced polyhedral) carboranesmixed hydride clusters of boron and carbon with BBB, BBC, or BCC faces. Each carbon atom in these cluster compounds has a hydrogen atom attached to it by a bond pointing away from the center of the cluster, but otherwise uses its three remaining valences to bond to the... 

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