The potential energy surfaces for the insertion reactions of germylene into XH n molecules have been characterized in detail using ab initio molecular orbital theory and density functional theory. The model system Ge(CH3)2 + XH n (X = C, N, O, F, Si, P, S, and Cl; n = 1−4) has been chosen for the present study. All the interactions involve the initial formation of a donor−acceptor complex, followed by a high-energy transition state, and then an insertion product. The agreement between MP2 and B3LYP results indicates that the latter provides an adequate theoretical level for further investigations of molecular geometries, electronic structures, and kinetic features of the germylene reactions. The following conclusions emerge from this work:  (i) the X−H insertion reactions of germylene occur in a concerted manner via a three-membered-ring transition state, and that the stereochemistry at the heteroatom X center is preserved; (ii) the stabilization energies of the germylene−XH n complexes increase in the order NH3 > H2O > PH3 > H2S ∼ HF > HCl ≫ SiH4 ∼ CH4; (iii) the order of reactivity for X−H bonds toward germylene insertion is Cl > F > S > O > P > N ≫ Si > C. In other words, the greater the atomic number of heteroatom (X) in a given row, the easier the insertion reaction of XH n hydrides and the larger the exothermicity. Moreover, the present study demonstrates that both electronic and steric effects play a major role in the course of insertion reactions of germylene into X−H bonds. This work also indicates that the chemical behavior of germylene should be more similar to that of silylene than to that of carbene species.

Complete geometry optimizations were carried out using density functional theory to study potential energy surfaces for the insertion of germylene into C−H bonds of methane. The GeXY + CH4 (GeXY = GeH2, GeCH2, GeH(CH3), Ge(CH3)2, GeHF, GeF2, GeHCl, GeCl2, GeHBr, and GeBr2) systems are the subject of the present study. All the stationary points were determined at the B3LYP/6-311G* level of theory. Our theoretical findings suggest that the computed structures of germylenes are in good agreement with the available experimental results, with the bond lengths and angles in agreement to within 0.04 Å and 1.0°, respectively. A configuration mixing model based on the theory of Pross and Shaik has been used to develop an explanation for the barrier height as well as the reaction enthalpy. Our theoretical findings suggest that the singlet−triplet splitting (ΔE st = E triplet − E singlet) of the GeXY species can be used as a guide to predict its activity for insertion reactions. Thus, the major conclusion that can be drawn from this work is as follows:  the more strongly π-accepting, the bulkier, or the more electropositive the substituents, the smaller the ΔE st of GeXY, the lower the activation energy, and the larger the exothermicity for the insertion of GeXY into saturated C−H bonds. In other words, it is the electronic factors, rather than steric factors, that play a decisive role in determining the chemical reactivity of the germylene species.

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.

We have chosen six ML 2 complexes, with a systematic variation in the ligands and metals, to investigate oxidative additions as well as reductive eliminations by using the MP2/LANL2DZ and the MP4SDTQ//MP2/LANL2DZ levels of theory. A qualitative model based on the theory of Pross and Shaik (Su, M.-D. Inorg. Chem. 1995, 34, 3829) has been used to develop an explanation for the barrier heights. Considering the geometrical effect, the substituent effect, and the nature of the metal center, the following conclusions emerge: for 14-electron ML 2 complexes, a smaller L-M-L angle and a better electron-donating ligand as well as a heavier transition metal center (such as Pt) should be a potential model for the oxidative addition of saturated C-H bonds. Conversely, a linear structure and a better electron-withdrawing ligand as well as a lighter transition metal center (such as Pd) would be a good candidate for reductive coupling of C-H bonds. The results obtained are in good agreement with the available experimental results and permit a number of predictions to be made. S0020-1669(97)00320-0 CCC: $15.00

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.