The reaction mechanism for the activation of C−H bonds by coordinatively unsaturated CpM(PH3)(CH3)+ (Cp = cyclopentadienyl; M = Rh, Ir) has been investigated by ab initio molecular orbital methods. Of the two possible mechanisms, an oxidative addition−reductive elimination process (path 1) and a σ-bond metathesis mechanism through a four-center transition state (path 2), only the former is found for the 16-electron Ir cation, while the Rh case might adopt the latter. The reaction trajectory of path 1 for the approach of CpM(PH3)(CH3)+ to methane and the transition state structure can be predicted on the basis of a frontier molecular orbital model that determines the orientation of attack of the CpM(PH3)(CH3)+ fragment on a doubly occupied canonical fragment molecular orbital of methane. From which, four kinds of reaction paths (paths A, B, C, and D) can be deduced due to the asymmetric nature of CpM(PH3)(CH3)+. Both MP2 and QCISD results suggest that path A, where the methane C−H bond breaks on the ancillary CH3 ligand side, is more favorable than other reaction paths kinetically and thermodynamically for both Rh and Ir cases. The calculational results strongly indicate that the reaction of the rhodium complex is intrinsically more difficult than that of the iridium complex. A qualitative model that is based on the theory of Pross and Shaik has been used to develop an explanation for the origin of the barrier height as well as the reaction enthalpy.

Complete geometry optimizations were carried out using density functional theory to study the potential energy surfaces for cycloaddition of germylene to the CC double bond of ethylene. The GeX2 + C2H4 (GeX2 = GeH2, Ge(CH3)2, Ge(NH2)2, Ge(OH)2, GeF2, GeCl2, GeBr2, and GeCH2) systems are the subject of the present study. All the stationary points were determined at the B3LYP/6-31G* level of theory. The major conclusions that can be drawn from this work are as follows:  (i) In contrast to the case of the carbene additions, a π-complex intermediate is formed between germylene and ethylene, which should play a key role in subsequent polymerization. (ii) On the basis of the results of the present study, it is apparent that germylene cycloadditions occur in a concerted, asynchronous manner. (iii) Germacyclopropanes, unlike cyclopropanes, are quite unstable compounds, reverting thermally to their precursors and then polymerizing rapidly, or even reacting with a second molecule of olefin to yield a cyclic compound. (iv) Considering the effect of substitution at the germanium center, our theoretical findings suggest that the cycloaddition of germylene with electropositive and/or bulky substituents is feasible from both a kinetic and a thermodynamic viewpoint. In contrast, germylenes bearing electronegative and/or π-donating substituents will tend not to undergo cycloadditions. Note that this conclusion is based upon the assumption that three-membered-ring germacyclopropane is the unique end product for germylene additions. (v) The cycloadditions of germylenes to alkenes are more endothermic (or less exothermic) than the same reactions of carbenes, reflecting the weaker Ge−C vs C−C bond.