Coupling of methylene and dihydrogen ligands in d(6) octahedral complexes

The rotational barriers of methylene and dihydrogen ligands were studied in the d(6) octahedral complexes Os(NH3)(5)(CH2)(2+) and Os(NH3)(5)(H-2)(2+), respectively, using the DFT-based B3LYP methodology. In both cases, the eclipsed conformation is found to be slightly more stable than the staggered conformation, but the energy barrier is very low (<1 kcal/mol). The coupling of the orientations of methylene and dihydrogen ligands attached to the same metal center was then studied in the trans-and the Cis-Os(NH3)(4)(H-2)(CH2)(2+) complexes. Geometry optimizations were performed at the B3LYP computational level and the energies recalculated at the MP2, MP4, and CCSD(T) levels. In both the cis and trans isomers the most stable conformations are those in which the pi-bonding interactions between the octahedral t(2g) set and the p(CH2) and sigma(H2)* acceptor orbitals are maximized. The strength of the coupling in the cis isomer is found to be more than twice that in the trans isomer, a result traced to stronger Os-H-2 bonding in the former. Despite this coupling, the rotational barriers of H-2 and CH2 remain low (congruent to 1 and 4 kcal/mol, respectively, at the CCSD(T)//B3LYP lever) because these processes can be achieved without going through the conformation disfavored on ii electronic grounds. Finally, the energy difference between the cis and the trans isomers is found to be very small (1.18 kcal/mol in favor of the former at the CCSD(T)//B3LYP computational level). These results are discussed with respect to the scarce experimental data available for carbene-dihydrogen d(6) octahedral complexes.