The 16 and 18 Electron Rule in Organometallic Chemistry and Homogeneous Catalysis By C. A. Tolman cENTRAL RESEARCH DEPARTMENT* EXPERIMENT A L STATION, E. I. DU PONT DE NEMOURS AND COMPANY, WILMINGTON, DELAWARE 19898, U.S.A. 1 Introduction There has been a great increase in interest and activity in recent years in