Two- and four-electron alkyne ligands in osmium - Cyclopentadienyl chemistry: Consequences of the π⊥→M interaction

The complexes Os(η5-C5H5)Cl{η2-HC≡CC (OH)R2}(PiPr3) (R = Ph (1a), Me (1b)) react with TlPF6 to give [Os(η5-C5H5){η2-HC≡CC( OH)R2}PiPr3)]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 1a to 2a, whereas the acetylenic bond lengths remain almost identical. Comparison of their 1H and 13C{1H} NMR spectra shows that the HC= 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(η5-C5H5)Cl(η2-HC=CR)(PH 3) (R = H (A), R = CH3 (ACH3)) and [Os(η5-C5H5)(η2-HC=CR)(PH 3)+ (R = H (B), R = CH3 (BCH3)) 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 GIAO method we succeed in qualitatively reproducing the experimental 1H and 13C chemical shifts. Both structural and spectroscopic changes can be explained by the participation of the acetylenic second π orbital (π⊥) 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 π∥ and the π⊥ 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.