Neutral boranes

Over 50 neutral boranes, B H , and an even larger number of borane anions have... 

B H +g. No neutral borane has yet been definitely established in this series but the known compounds BgHie and BioHig may prove to be hypho-hormcs and several adducts are known to have hypho-stmctures (pp. 171-2). 

Further checks, uhich can readily be verified from the equations of balance, are (a) the number of atoms in a neutral borane molecule = 2(s + I + y + x), and (b) there are as many framework electrons as (here are atoms in a neutral borane B H , since each BH group supplies 2 electrons and each of the m — n) "extra H atoms supplies I electron, making n + m in all. 

Neutral carboranes and boranes react with transition-metal complexes forming metallocarboranes or metalloboranes, respectively. However, most metallocarboranes and metalloboranes are prepared from transition-metal halides and anionic carborane and borane species ( 6.5.3.4) or by reacting metal atoms and neutral boranes and carboranes. These reactions are oxidative addition reactions ( 6.5.3.3). 

The same polyhedra serve as the basis for the structures of nido and arachno compounds, too, although for these boranes and carboranes the polyhedra are incomplete. Nido structures are adopted by neutral boranes BjjHw j.4, carboranes CBy 4Hn.. 3, CsBji—... 

The vacant edge-base bond in BgHio (I-N6) involves 2 five-coordinate borons probably the low coordination of the B—B bond (unique among neutral boranes) accounts for both the capacity of BaHjo to act as a Lewis base in accepting a variety of Lewis acids (63) as well as the short bond length. 

When more data are accumulated, the various borane anions, carboranes, and neutral boranes should only be compared with each other, rather than all together as in Table VII. 

NEUTRAL BORANE COMPLEXES 24.2.1 General Considerations and Stability... 

Since the discovery of NH3-BF3 by Gay-Lussac and Thenard12 in 1809, many neutral borane complexes have been described, mainly with N, P, As, O and S donor molecules. Their stabilities vary considerably many are formed at low temperatures only, whereas in other cases... 

The stability and equilibrium properties of neutral borane complexes are not well known. Instead, the recent chemistry of the borane complexes is predominantly characterized by preparative and structural investigations. This is even more obvious for the charged borane complexes. 

Prom neutral borane complexes via proton abstraction ... 

The hypho (Gr.. net ) series of boranes, with electronic formula 2n + 8, has beer suggested to augment the closo, nido, and arachno series. Although no neutral boranes fit this scheme, some borane derivatives do. It is also passible to construct units consisting of more than one of the above types. These are called cmjuncio (Latin, joined subunits ) structures. 

B H ct complexes containing neutral borane ligands have recently been established. Boranes such as BH3 are Lewis acids but their base adducts such as BH3 - PR3 are potential a ligands analogous to silanes. The X-ray structures of group 6 pentacarbonyl analogues such as W(CO).-,(BH i PMe3) clearly show monodentate r 2-B H coordination (128). 

Complex 26 is among the first examples of intermolecular coordination of a single B-H bond in a neutral borane to a transition metal, and these species can be regarded as models for alkane coordination. The crystal structures of borane complexes such as 26 are of interest to compare with those for other octahedral a complexes, particularly... 

Table 1 shows examples of closo-, nido-, arachno- and hypho-boianes containing 6 through 13 skeletal bonding electron pairs. Where neutral borane example are not available, ionic species or heteroboranes are cited. 

The larger nido and arachno cages are usually obtained via degradation (in one or more steps) of icosahedral carboranes, as previously mentioned, but the smaller members are typically prepared in reactions of alkynes with neutral boranes, as in the synthesis of 2,3-RR C2B4H8 (Fig. 5-14). However, the nido-2,4-QBJHf carbons apart ion can be obtained by removal of an apex boron from closo-2,4-QB5B7 (Fig. 5-10d).8... 

As an example, let us consider neutral boranes with composition BpHp+g. Suppose the number of hydrogen bridge atoms is denoted by s, the number of terminal BH or BH2 groups by x, the number of three-centre bonds BBB (both open and central) by t, and the number of two-centre bonds B—B by y. 

Examples oftetra-coordinate boronium ions (41) have been known for many years. The quest for low-coordinate, neutral borane compounds also stimulated interest in the formation of three-coordinate borenium ions (42) and two-coordinate borinium ions (43). ... 

Other examples of new B B bridge formation involve the condensation of hydroborate anions with neutral boranes to form large boranes or borane anions. Bo-rane will add to the anions [B H,] , [BjHg]", and [B H,] to form the species [BjHjj] , [BjH,] and respectively " ... 

The neutral borane molecules, interesting and complex as they are, represent only the beginning of the chemistry of boron-hydrogen com-... 

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