Ab initio quantum mechanical calculations were performed on model reactions to analyze the behavior of intermolecular versus intramolecular C-H bond activation in zirconium, rhodium, and iridium complexes. Intermolecular reactions (inter) were modeled by (Cp)(X)M + CH 4 f (Cp)(X)M(CH 3 )(H), with (M, X) ) (Zr, Cl), (Rh, PH 3 ), and (Ir, PH 3 ) and Cp ) C 5 H 5 . Intramolecular reactions involving the Cp ring (intra-Cp) were modeled by (X)M(CpR′H) f (X)M(CpR′)(H), while those involving the phosphine (intra-P) (Rh and Ir only) were modeled by (Cp)M(PH 2 R′H) f (Cp)M(PH 2 R′)(H), with R′ ) CH 2 and CH 2 CH 2 . It is found that the thermodynamic exothermicity follows the sequence inter > intra-P > intra-Cp with decreasing differences as the ring size increases. The "strain" energy for the intra-Cp reactions is less for Zr complexes than it is for the corresponding Rh or Ir complexes. Agostic Ir and Zr intermediates were optimized and are bound by 6.7 and 1.3 kcal/mol, respectively, but the intermolecular reactions have negligible kinetic barriers at the MP2 level. The intra-P (Rh and Ir) reactions beginning with reactants in their singlet states also have virtually no kinetic barrier at the MP2 level, even for systems with significant "strain" energy. The barrier for the intra-Cp reaction decreases as the R′ fragment increases and is much smaller for the Zr complexes than for the corresponding Rh or Ir complexes.

Ab initio quantum mechanical calculations were used to examine models for the reaction [η 3 -HB(X) 3 ](CO)M(η 2 -CH 2 CH 2 ) f [η 2 -HB(X) 3 ](CO)M(η 2 -CH 2 CH 2 ) f [η 3 -HB(X) 3 ](CO)M(H)-(CHCH 2 ), for which it is known that the equilibrium lies toward the hydridovinyl product for iridium, with X ) 3-trifluoromethyl-5-methylpyrazol-1-yl, and lies toward the η 2 -ethene reactant for rhodium, with X ) 3,5-dimethylpyrazol-1-yl, and most other related systems. The ligand models tested correspond to X ) NHNH 2 , NHNCHF, N 2 C 3 H 3 (pyrazol-1-yl), N 2 C 3 H 2 F (3-fluoropyrazol-1-yl), N 2 C 3 H 2 F (5-fluoropyrazol-1-yl), N 2 C 3 H 2 CH 3 (3-methylpyrazol-1-yl), and N 2 C 3 H 2 CF 3 (3-(trifluoromethyl)pyrazol-1-yl). For the iridium complexes, the restricted Hartree-Fock (RHF) optimized geometries are similar to the Møller-Plesset second-order (MP2) ones and the energy calculations at the MP2//RHF, MP2//MP2, and MP4SDQ//MP2 levels give reasonable results. For the rhodium complexes, although the RHF energies appear to be in qualitative accord with the calculated results at the MP2// MP2 and MP4SDQ//MP2 levels, the MP2//RHF energies are not in agreement with these results since the RHF geometries of (η 3 -HBX 3 )(η 2 -ethene)rhodium complexes are very sensitive to the electronic environment. In the d 8 η 2 -ethene complexes, Rh favors η 2 -pyrazolylborate over η 3 -pyrazolylborate complexes by -0.7 to -9.9 kcal/mol and the barrier from the hydridovinyl complex is only 12 kcal/mol. Thus, nearly all Rh complexes will exist as (η 2 -ethene)(η 2 -pyrazolylborate) complexes. In contrast, Ir favors the η 3 -pyrazolylborate over η 2 -pyrazolylborate in all complexes and endothermicty to the hydridovinyl complex is reduced by about 20 kcal/mol. The ethene complexes are the most sensitive to the steric properties of the pyrazolylborate ligands. Thus, in the most sterically hindered Ir complexes, the hydridovinyl complex finally becomes more stable than its η 2 -ethene isomer.

A wide range of values for the exothermicity of the oxidative addition of methane to (cyclopentadienyl)-rhodium carbonyl have been reported at the MP2 level of theory. These energies are recalculated using several different basis sets which reproduce previous results and provide evidence for the inadequacy of some commonly used basis sets. The calculations show that the magnitude of the exothermicity of this reaction is particularly sensitive to the metal 5s orbital. With a metal basis set which includes proper representation of the (n + 1)s and -p metal orbitals, the exothermicity is also calculated at the MP3, MP4(SDQ), and QCISD levels. The best results show that the MPX perturbation series does not converge well and that the MP2 results in any basis set are only qualitative. IntroductionOver the last few years, the activation of methane by RhCp(CO), reaction 1, has been studied extensively both experimentally 1,2 and theoretically. 3-7 However, the experimental exothermicity of the reaction is not known accurately and the theoretical value is the subject of some disagreement.At the restricted Hartree-Fock (RHF) level of theory, the reaction is endothermic and all authors agree on the crucial importance of electron correlation. Møller-Plesset second-order perturbation theory (MP2) is the most widely used method for these calculations. Song and Hall 4 used a MP2 energy calculation at the Hartree-Fock optimized geometry to predict an exothermicity of -31.0 kcal/mol for the reaction. Recently, Musaev and Morokuma 5 performed MP2 geometry optimizations, recalculated the energy adding polarization functions on all first-row atoms, and obtained -16.4 kcal/mol. More recently, Siegbahn 6 reported a value of -18.8 kcal/mol at the MP2 level. Jiménez 7 performed a number of MP2 calculations at both RHF and MP2 geometries in a variety of basis sets and found values ranging from -28.7 to -35.9 kcal/mol. In this work, the difference between the MP2 energy at the RHFoptimized geometry and that at the MP2-optimized geometry was less than 3 kcal/mol, while the MP2 difference in exothermicity due to the polarization functions on first-row atoms was less than 5 kcal/mol. It is particularly curious that the calculated exothermicities cluster around values which differ by approximately 15 kcal/mol. This large difference leads us to believe that there are significant, but poorly understood, differences in the basis sets used in the calculations on this system.In recent studies of the oxidative addition of dihydrogen to transition metals, we noted that the energy of the system was unexpectedly sensitive to the representation of the outer (n + 1)p orbital. This result prompted us to calculate improved (n + 1)p orbitals for all the transition metals. 8 Here, we will show that reaction 1 is also very sensitive to the representation of the outer (n + 1) orbitals and in particular to the (n + 1)s orbital. We have also calculated the exothermicity at several higher levels of electron correlation, since it is well-known that MP2 calculatio...

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