Hypercarbon systems

In introducing the subject in Section 1.2, we dehne some of the terms we shall be using. In Section 1.3, we illustrate the various types of hypercarbon compounds now known. Since we shall rely heavily on the 3c-2e bond concept in their bonding, and since its usefulness is perhaps less widely appreciated in organic chemistry than in inorganic or organometallic chemistry, we devote Section 1.4 of this introductory chapter to discussion of that concept and illustrate its value for selected systems. We also demonstrate the relevance and value of some simple MO arguments applied to hypercarbon systems (Sections... 

Before exploring the various bonding situations that occur in hypercarbon systems, we illustrate the structures of some representative examples, grouped according to type in Figures 1.1-1.6. 

Thus far, our discussion of the bonding in hypercarbon systems has focused on various types of three-center bonding situations, noting the importance of... 

Although the main concern of this book is to survey the structures and bonding of hypercarbon systems, it is appropriate to note briefly the routes by which carboranes have been synthesized, and methods by which one carborane can be converted into another. Section 3.7 outlines the types of reaction that can occur at the highly coordinated carbon atoms of carboranes. 

K-bonding electrons to form the metal-carbon bonds their structures may be regarded as mixed metal-carbon clusters, the shapes of which clearly reflect the numbers of electrons available as do the mixed boron-carbon cluster shapes of carboranes. All are members of the same family of hypercarbon systems. 

The value of drawing comparisons between hypercarbon systems and analogous isoelectronic boranes has been illustrated in earlier chapters in this book. 

It was the study of hypercarbon-containing nonclassical carboca-tions that allowed us to firmly establish carbon s ability in a hydrocarbon system to bind simultneously with five (or six or even seven) atoms or groups. It should be emphasized that carbocations represent... 

To show how hypercarbon compounds are closely related to many classically bonded systems and aromatic systems, and are not exotic species remote from mainstream organic chemistry. 

Figure 1.6. Agostic systems containing carbon-hydrogen-metal 3c-2e bonds ( denotes hypercarbons).

The dative two-center bonds in the adducts formed in such solutions make much better use of the fourth metal orbital than do the three-center Al—C—Al bridge bonds they replace. For this reason, in studies of systems in which metal atoms are bridged by hypercarbon atoms, it is advisable to avoid or restrict exposure of the system to Lewis bases, though in this chapter we shall consider some associated metal alkyls that remain associated even in the presence of an excess of Lewis base. 

Compound 48 and the ferrocenyl-gold""" and ruthenocenyl-gold " compounds 49 provide interesting examples of a hypercarbon atom that is not only part of an aromatic cyclopentadienyl ring system, in which it is bonded to two other carbon atoms, but also bonds simultaneously to the sandwiched iron atom and (by a 3c-2e bond) to the two coinage atoms. The related dication 50 has also been isolated. " ... 

Collectively, these systems show how a hypercarbon atom occupying a pyramidal site above a ring of three, four, or five boron atoms (themselves occupying equatorial sites when n = 5,6, or 7, or tropical sites when n = 10 or 12) forms C-B bonds of increasing length and decreasing bond order as n or k increases. The data also show how the charges on the CH and BH units vary with n and k, and the extent to which the skeletal electron distribution in these systems varies as one or two BH units in the parent cluster are replaced by CH units. 

This chapter has shown how hypercarbon atoms and metal atoms can form mixed metal-carbon cluster systems. Our concern has been mainly with... 

In this chapter, we successively review reactions of electrophiles, coordinatively unsaturated metal compounds, carbenes, nitrenes, and heavy analogs of carbenes (silylenes, germylenes, stannylenes) with C-H and C-C bonds. Discussion of some electrophilic reactions of ti-donor systems is also included, along with Sn2 reactions. Tlie emphasis in all these discussions is centered on the involvement of hypercarbon intermediates (or transition states) of the reactions. 

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