Theoretical studies of inorganic and organometallic reaction mechanisms .10. Reversal in stability of rhodium and iridium eta(2)-ethene and hydridovinyl complexes
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Ab initio quantum mechanical calculations were used to examine models for the reaction [3-HB(X)3](CO)M(2-CH 2CH2) [2-HB(X)3](CO)M(2-CH 2CH2) [3-HB(X)3](CO)M(H)-(CHCH2), for which it is known that the equilibrium lies toward the hydridovinyl product for iridium, with X = 3-trifluoromethyl-S-methylpyrazol-1-yl, and lies toward the 2-ethene reactant for rhodium, with X = 3,5-dimethylpyrazol-1-yl, and most other related systems. The ligand models tested correspond to X = NHNH2, NHNCHF, N2C3H3 (pyrazol-1-yl), N2C3H2F (3-fluoropyrazol-1-yl), N2C3H2F (5-fluoropyrazol-1-yl), N2C3H2CH3 (3-methylpyrazol-1-yl), and N2C3H2CF3 (3-(trifluoromethyl)pyrazol-1-yl). For the iridium complexes, the restricted Hartree-Fock (RHF) optimized geometries are similar to the Mller - 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 (3-HBX3)(2-ethene)rhodium complexes are very sensitive to the electronic environment. In the d8 2-ethene complexes, Rh favors 2-pyrazolylborate over 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 (2-ethene)(2-pyrazolylborate) complexes. In contrast, Ir favors the 3-pyrazolylborate over 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 2-ethene isomer.
