Dissociative and nondissociative pathways in the endo to exo isomerization of tetramethyl-o-xylylene complexes of ruthenium and osmium, ML3{η4-o-C6Me4(CH2)2} (M = Ru, L = PMe3; M = Os, L = PMe3, PMe2Ph).: Formation of hexamethylbenzene-1,2-diyl complexes by ligand addition to the exo-osmium complexes

Treatment of the (eta(6)-hexamethylbenzene)ruthenium(II) and -osmium(II) salts [M(O-2-CCF3)L-2(eta(6)-C6Me6)]PF6 (M = Ru, L = PMe3; M = Os, L = PMe3, PMe2Ph) in the presence of L with KO-t-Bu gives exclusively the endo- (tetramethyl-o-xylylene)metal(0) complexes ML3{eta(4)-endo-o-C6Me4(CH2)(2)}, endo-1, -2, and -3, respectively, in high yield; these-are protonated by an excess of triflic acid (CF3SO3H, TfOH) to give the (eta(6)-hexamethylbenzene)metal(II) salts [ML3(eta(6)-C6Me6)](OTf)(2) [M = Ru, L = PMe3 (4); M = Os, L = PMe3 (5); M = Os, L = PMe2Ph (6)). Complexes 4-6 revert to endo-1-3 on treatment with. KO-t-Bu, whereas for M=Ru, L=PMe2Ph the complexes [ML3(eta(6)-C6Me6)](2+) and [M(O2CCF3)L-2(eta(6)-C6-Me6)](+)/L react with KO-t-Bu to give exclusively the exo isomer, Ru(PMe2Ph)(3){eta(4)-exo-o-(CH2)(2)C6Me4} (exo-7). The endo complexes 1-3 are converted quantitatively into the corresponding exo isomers in toluene in the temperature range 65-106 degrees C, the process being first order in endo complex. Kinetics studies in the presence of PMe3 (for 1 and 2) or PMe2-Ph (for 3) indicate that two pathways are available: one depends on initial dissociation of L and proceeds through a bis(ligand) intermediate or intermediates, e.g., ML2{endo-o-C6-Me4(CH2)(2)} and ML2{exo-o-(CH2)(2)C6Me4}, and the other does not. The dissociative mechanism is predominant for M = Ru, L = PMe3, whereas the nondissociative or direct mechanism plays the dominant, possibly exclusive, role for M = Os, L = PMe3. The osmium(0) compound exo-2 adds PMe3 irreversibly to give the sigma-bonded (hexamethylbenzene-1,2-diyl)osmium(II) complex Os(PMe3)(4){kappa(2)-o-(CH2)(2)C6Me4} (8), whereas the corresponding PMe2Ph derivative 9 is in equilibrium with exo-3 and PMe2Ph and cannot be isolated; the ruthenium(0) compound exo-1 is inert toward PMe3. Density functional calculations on the model compounds ML3-{eta(4)-exo-o-(CH2)(2)C6H4} and ML4{kappa(2)-o-(CH2)(2)C6H4}(M = RU, Os; L = PH3) correctly reflect the observed stability order Os > Ru for the diyl complex but predict the latter to be more stable than the eta(4) complex far both elements. In this case, the usual computational simplification of replacing a tertiary phosphine by PH3 is probably unjustified, The molecular structures of the eta(4) complexes endo-3, exo-3, and exo-1 and of the diyl complex 8 have been determined by X-ray crystallography. The endo- to exo-o-xylylene isomerizations are compared with the intramolecular migrations that occur in Fe(CO)(3)(eta(4)-polyene) and Cr(CO)(3)(eta(6)-substituted-naphthalene) complexes.