Linkage isomerism in complexes of o-xylylene: Density functional study of the structure and bonding in endo- and exo-Ru(PH3)(3)(o-xylylene)

The bonding in eta(4) complexes of butadiene (C4H6) and o-xylylene (C8H8) with zerovalent ruthenium is examined using density functional theory. The ruthenium-butadiene bond is shown to be stabilized principally by charge transfer from the metal to the ligand, as a result of which the central C-C bond is contracted relative to that in the free ligand. Back-bonding is further enhanced by a rotation of the substituents at the terminal carbon atoms, resulting in a stabilization of the ligand LUMO and also enhanced overlap of metal and ligand-based orbitals. The major features of the bonding in butadiene complexes are extended to the o-xylylene ligand, which contains two fused butadiene fragments and, hence, two distinct eta(4) coordination sites. The exo isomer of Ru(PH3)(3)(o-xylylene) is found to be more stable than its endo counterpart, principally due to an aromatization of the six-membered ring when coordination occurs at the exo site. In addition, the stabilizing influence of the rotation of the terminal carbon atoms is less pronounced in the endo isomer, due to the steric constraints of the six-membered ring.