These notes are based on lectures on molecular orbital theory that we have presented at the University of Copenhagen and Columbia University. They were designed primarily for advanced-undergraduate and first-year graduate students as an introduction to molecular orbital theory.

It is apparent that the molecular orbital theory is a very useful method of classifying the ground and excited states of small molecules. The transition metal complexes occupy a special place here, and the last chapter is devoted entirely to this subject. We believe that modern inorganic chemists should be acquainted with the methods of the theory, and that they will find approximate one-electron calculations as helpful as the organic chemists have found simple Hückel calculations. For this reason, we have included a calculation of the permanganate ion in Chapter 8. On the other hand, we have not considered conjugated pi systems because they are excellently discussed in a number of books.

Our intuitive approach in the use of symmetry methods is admittedly nonrigorous and therefore will be unsatisfactory to purists, but we believe this is the best way to introduce symmetry ideas to the majority of students. Once the student has learned how to use symmetry methods, it will be easier for him to appreciate more formal and rigorous treatments.

Several reprints of papers on molecular orbital theory are included in the back of the book. The papers treat a substantial number of the important molecular geometries. The reader should be able to follow the discussions after reading through the lecture notes.

We thank our colleagues in New York and Copenhagen for help with the manuscript. We gratefully acknowledge the help of Dr. Arlen Viste and Mr. Harold Basch in preparing Appendix 8-B. Finally, it is a pleasure to acknowledge the expert assistance of Mrs. Diane Celeste in preparing the final manuscript.

C. J. BALLHAUSEN, Kobenhavn
HARRY B. GRAY, New York
October 1964