Hypercarbon atom

This book is concerned with an important area of organic (i.e., carbon) chemistry that has developed enormously over the past half-century, yet is still neglected in many organic textbooks. This is the chemistry of compounds in which carbon atoms are covalently bonded to more neighboring atoms than can be explained in terms of classical two-center, electron-pair bonds. Such carbon atoms are referred to as hypercarbon atoms (short for hypercoordi-nated carbon atoms ) because when first discovered, their coordination numbers seemed unexpectedly high. 

To illustrate the wide and developing scope of hypercarbon chemistry by illustrating the variety of compounds now known to contain hypercarbon atoms (carbocations,organometallics, carboranes, metal-carbon cluster compounds," and metal carbides ).They include bridged metal alkyls such as alkyl-lithium reagents (LiR) in which the hypercoordinated nature of the metal-attached carbon atoms, and the roles that the metal atoms play in their chemistry, are often overlooked. 

Figure 1.2. Carbocations containing hypercarbon atoms, (a) Carbocations (b) carbocationic intermediates or transition states ( 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. 

The hypercarbon atom environment in this compound, with one silicon atom, two hydrogen atoms, and two copper atoms in the carbon coordination sphere, with a Cu-C-Cu bond angle of 74°, is consistent with the formation of three 2c-2e bonds to the silicon and hydrogen atoms, and a 3c-2e bond to the metal atoms. Hiis open cyclic structure, which may be contrasted with the more compact tetrahedral structures of typical tetrameric lithium alkyls, suggests that the metal atoms are sp hybridized, unable to make use of as many AOs as lithium atoms can. 

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. " ... 

X-ray diffraction studies on yttrium and ytterbium" compounds (C5H5)2MR2A1R2 have established their structures as of type 58 with the characteristically acute M-C-Al bond angle at the hypercarbon atom that shows involvement in a 3c-2e bond to the metal atoms while bonding normally, by three 2c-2e bonds, to the methyl hydrogen atoms. 

Bulky alkyl groups CH2R (R = Ph, CMca, SiMe, or CMo Pli), which have no hydrogen atoms attached to the carbon or silicon atom in the p position, have been widely used to probe the alkyl chemistry of transition metals Uieir bulk protects the metal from nucleophilic attack, while the absence of 3 hydrogen reduces the risk of decomposition by metal hydride formation and alkene elimination. The use of such ligands attached to manganese has provided examples of hypercarbon atoms bridging pairs of metal atoms that are worthy of brief mention here. 

In this chapter we have shown that hypercarbon atoms play important roles in the alkyl and aryl derivative chemistry of relatively electropositive metals. 

In the second type of mixed hydride, hypercarbon atoms feature alongside boron atoms in the polyhedral molecular skeleton. Most of these mixed hydrides contain more boron atoms than carbon atoms, and their formulae and structures can be understood simply by isoelectronic replacement of B by C, B by C+, or BH units by C atoms in the parent borane. They are therefore known as carbaboranes, though the shorter name carboranes, coined soon after their discovery, "- is that most often used for this important category of compound and will be used here. Their now well-documented chemistry, " particularly structural and bonding aspects, is the concern of the present chapter. 

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. 

However, the external (CH)4 link between the carborane hypercarbon atoms shows the alternation in C-C bond distances (1.48,1.34,1.45,... 

That exo multiple bonding to an orz/zo-carborane cage hypercarbon atom is possible (but with orientational constraints that prevent its occurrence in benzocarborane, and with fascinating imphcations for the use of ortho-carborane as a 7t-acidic probe by which to measure the n-donor capacity of substituents) has been demonstrated by a series of studies on derivatives of phenyl-orz/io-carborane PhCBioHioCX bearing assorted zi-donor C-attached... 

Higgins and Roberts/ using simple MO arguments, some years before icosahedral borane anions and carboranes were discovered. Tlie prospect that supraiocosahedral carboranes will provide yet more exotic sites for hypercarbon atoms remains high. 

METAL CLUSTERS INCORPORATING CORE HYPERCARBON ATOMS... 

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

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