N-heterocyclic carbenes, such as the example shown here, have become increasingly import...

N-heterocyclic carbenes, such as the example shown here, have become increasingly important in both main group and transition-metal chemistry. For example, the first report of a stable neutral molecule having a boron–boron double bond used this N-heterocyclic carbene to bond to each of the borons in a product of the reaction of RBBr₃ with the powerful reducing agent KC₈ (potassium graphite) in diethylether solvent. (See Y. Wang, B. Quillian, P. Wei, C. S. Wannere, Y. Xie, R. B. King, H. F. Schaefer, III, P. v. R. Schleyer, G. H. Robinson, J. Am. Chem. Soc. , 2007 , 129 , 12412.)

a. What was the structure of this first molecule with a B “ B bond? What evidence was cited for a double bond?

b. If RSiCl₄ was used in place of RBBr₃, an equally notable silicon compound was formed. What was this compound, and what parallels were noted with the reaction in Part a? (See Y. Wang, Y. Xie, P. Wei, R. B. King, H. F. Schaefer III, P. v. R. Schleyer, G. H. Robinson, Scienc e , 2008 , 321 , 1069.)