Singlet–triplet splitting and the activation of CH bond for (η 5 ‐C 5 H 5 )M(CO) isoelectronic fragments: A theoretical study

Abstract

A density functional study was used to investigate potential energy surfaces of the oxidative addition reaction CpM(CO)+CH4CpM(CO)(H)(CH3) (M=Ru−, Os−, Rh, Ir, Pd+, and Pt+). A qualitative model which is based on the theory of Pross and Shaik was applied to develop an explanation for the barrier heights. As a result, our theoretical findings suggest that the singlet–triplet splitting (ΔEst=Etriplet−Esinglet) of the CpM(CO) species can be a guide to predict its reaction activity and enthalpy for oxidative additions. A better linear correlation was found between the latter two quantities. Considering the nature of the metal center, the following conclusions therefore emerged: For the 16-electron CpM(CO) system, a heavier transition-metal center (i.e., the third-row) will lead to a smaller ΔEst and, in turn, will facilitate the oxidative addition reactions to alkane CH bonds. In contrast, a lighter transition-metal center (i.e., the second row) will result in a larger ΔEst and then tend to undergo the reductive elimination reactions of the CH bond-forming. The results obtained are in good agreement with the available experimental results and allow a number of predictions to be made. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 961–971, 1998