Bridged Associated Metal Alkyls

Hypercarbon Chemistry, Second Edition. George A. Olah, G. K. Surya Prakash, Kenneth Wade, Arpad Molnar, Robert E. Williams. 

5 kcal mol stronger than its two-center Al-C terminal counterpart in structure 2, sufficient to make 1 the thermodynamically preferred species at normal temperatures, but not enough to prevent cleavage of the dimer by Lewis bases such as ethers or tertiary amines, in which trimethylaluminum dissolves as the monomer [Eq. (2.1)]  

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. 

In order for it to function as a Lewis acid catalyst, the dimer AbCU can undergo the less energetic opening of only one of the two chlorine bridges to form the half-bridged structure 3 [Eq. (2.2)] in which the three-coordinate metal atom can function as a strong Lewis acid. Similar half-bridged structures in dimeric aluminum alkyls and other associated metal alkyls can be expected to play roles in their reactions with bases. 

Although the coordination numbers are unexceptional, and strictly do not justify treatment of these systems as examples of hypercoordinate carbon, we shall see that the bonding of their carbon atoms is very similar to that of the hypercoordinate atoms in associated dialkyls, in that three carbon valences are essentially occupied in bonds within the bridging ligand, while the remaining valency is used to form a three-center metal-carbon-metal bond. 

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