I gratefully acknowledge the superb work of my coworkers whose efforts have produced the results presented in these pages. I have also benefited personally from their friendship which has made our work together most enjoyable.

The group's efforts have also benefited from the outstanding technical assistance of Mr. Hugh A. Craig (DuPont), Mr. Michael Kline (DuPont), and Ms. Usha Sharma (DuPont).

The help of William Marshall in assembling the data for over 300 X-ray structures was invaluable.

Finally, I thank Dr. Markus Unverzagt and Dr. P. Michele Arduengo for their suggestions on these pages.