Two- and four-electron alkyne Ligands in osmium-cyclopentadienyl chemistry:: Consequences the π⊥→M interaction

The complexes Os(eta(5)-C5H5)Cl{eta(2)-HCequivalent toCC(OH)R-2}((PPr3)-Pr-i) (R = Ph (1a), Me (1b)) react with TlPF6 to give [Os(eta(5)-C5H5){eta(2)-HCequivalent toCC(OH)R-2}((PPr3)-Pr-i)]PF6 (R = Ph (2a), Me (2b)). The structures of 1a and 2a have been determined by X-ray diffraction. The comparative study of the data reveals a shortening of the Os-C(alkyne) distances on going from la to 2a, whereas the acetylenic bond lengths remain almost identical. Comparison of their H-1 and C-13{H-1} NMR spectra shows that the HCequivalent to proton resonances and the chemical shifts of the acetylenic carbon atoms of 2a and 2b are substantially shifted toward lower field than are those of 1a and 1b. DFT calculations were carried out on Os(eta(5)-C5H5)Cl(eta(2)-HCequivalent toCR)(PH3) (R = H (A), R = CH3 (A(CH3))) and [Os(eta(5)-C5H5)(eta(2)-HCequivalent toCR)(PH3)](+) (R = H (B), R = CH3 (B-CH3)) model systems in order to study the differences in bonding nature of the two parent alkyne complexes, 1 and 2. Calculations give geometries very close to the X-ray-determined ones, and by using the GIRO method we succeed in. qualitatively reproducing the experimental H-1 and C-13 chemical shifts. Both structural and spectroscopic changes can be explained by the participation of the acetylenic second pi orbital (pi(perpendicular to)) in the metal-alkyne bonding. As we go from 1 to 2 or from A to B, the extraction of the chloride ligand transforms the 2-electron-donor alkyne ligand to a 4-electron-donor ligand, with both the pi(parallel to) and the pi(perpendicular to) orbitals donating to the metal and stabilizing the otherwise 16-electron unsaturated complex 2. Calculations also predict an increase of dissociation energies of the alkyne, and an enhancement in the energy of rotation of the alkyne, for complex B. Finally, Bader's atoms in molecules (AIM) analysis shows that differences in coordination nature are also reflected in the topological properties of electron density.