Boron cage compounds

Reduction of boron halides might be expected to produce boranc. BH,. However, il is impossible to isolate the monomer, all syntheses resulting in diborane. 

Although BH) exists in the form of Lewis acid-base adducts and as a presumable intermediate in reactions of diborane. only trace quantities of the free molecule have been detected. The equilibrium constant fur dimerization is approximately tt) and the enthalpy of dissociation of the dimer to the monomer is about + 150 kJ mol-1 or slightly more.120 

Diborane is the simplest of the boron hydrides, a class of compounds that have become known as electron deficient. They are electron deficient only in a formal sense—there are fewer electrons than required for ail of the adjacent atoms to be held together by electron-pair covalent bonds. The compounds, in fact, are good reducing 

1IV For a discussion of the synthetic chemistry of B—H compounds, sec Shore, S. G. In Rings. Clusters, and Polymers of the Main Group Elements. Cowley, A. H.. Ed. ACS Symposium Scries 232. American Chemical Society Washington. DC, 1983. 

The earliest attempt at rationalizing the dimerization of borane invoked resonance in a valence bond (VB) context  

Note that [Ns is true whether the r-rr model or a bent-bund model is employed for the double 

FIGURE 9.2 Some boron hydride cage compounds. 

The principal ways in which P atoms can be associated with boron cage compounds can be represented as in Equation 9.84, where B represents a boron polyhedron consisting of atoms together with an appropriate number of H atoms. 

Situation (9.84a) involves external P atoms linked directly to the B/H cage, while (9.84b) represents replacement of some B atoms in the original cage by P atoms. In both cases B-P linkages are present. 

In another situation (9.84c), the P atom can be linked to another atom X which has replaced a B atom in the original cage. In Equation 9.84d, the P atom is linked to the boron cage through a metal atom, and in Equation 9.84e through some other atom X. Boron and phosphorus reside in separate ions in case (9.84f). Examples of all these types of compound are known. 

Numerous reactions between phosphines and the boron hydrides are now known. Some of these reactions involve the breaking of B-H bonds with the elimination of hydrogen (9.85, 9.86), while others involve the break-up of the complex boron cage into smaller units (9.87-9.90). Non-phosphorus ligands of basic character also form complexes in this latter way [35]. 

---

Boranes are typical species with electron-deficient bonds, where a chemical bond has more centers than electrons. The smallest molecule showing this property is diborane. Each of the two B-H-B bonds (shown in Figure 2-60a) contains only two electrons, while the molecular orbital extends over three atoms. A correct representation has to represent the delocalization of the two electrons over three atom centers as shown in Figure 2-60b. Figure 2-60c shows another type of electron-deficient bond. In boron cage compounds, boron-boron bonds share their electron pair with the unoccupied atom orbital of a third boron atom [86]. These types of bonds cannot be accommodated in a single VB model of two-electron/ two-centered bonds. 

Figure 2-60. Soine examples of electron-deficient bonds a) diborane featuring B-H-B bonds b) diborane in a tentative RAMSES representation c) the orbital in a B-B-B bond (which occurs in boron cage compounds),...

There are a number of other important boron cage compounds as well as structurally similar metal atom cluster molecules that are not closed polyhedra. Generally, these may be regarded as derived from closed polyhedra by removal of one or two vertices. The diagram below illustrates how removal of one or two vertices generates the so-called nido and arachno relatives of a closo octahedral structure. 

Boron Cage Compounds 789 Metal Clusters 807 Conclusion 819... 

Potentlometrlc studies of chelates of phenylboronlc acid and pent OSes in acid solution showed only the presence of 1 1 species. FAB-m.s. has been used to examine negatively charged boronate cage compounds formed by boronic acids and trifunctional molecules such a. sugars and nucleosides on the probe tip the method is useful for th ... 

Triols, sugars, nucleosides, etc. Reaction with boronic acid to form negatively charged boronate cage compounds Enhanced sensitivity in the negative-ion mode via pre-formed ions 61... 

---