DISSOCIATIVE AND NONDISSOCIATIVE PATHWAYS IN THE ENDO TO EXO ISOMERIZATION OF TETRAMETHYL-O-XYLYLENE COMPLEXES OF RUTHENIUM AND OSMIUM, ML3(ETA(4)-O-C6ME4(CH2)(2)) (M = RU, L = PME3, M = OS, L = PME3, PME2PH) - FORMATION OF HEXAMETHYLBENZENE-1,2-DIYL COMPLEXES BY LIGAND ADDITIONTO THE EXO-OSMIUM COMPLEXES

Treatment of the (eta(6)-hexamethylbenzene)ruthenium(II) and -osmium(I I) 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; thes e-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, where as for M=Ru, L=PMe2Ph the complexes [ML3(eta(6)-C6Me6)](2+) and [M(O2C CF3)L-2(eta(6)-C6-Me6)](+)/L react with KO-t-Bu to give exclusively th e exo isomer, Ru(PMe2Ph)(3){eta(4)-exo-o-(CH2)(2)C6Me4} (exo-7). The e ndo 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 pre sence of PMe3 (for 1 and 2) or PMe2-Ph (for 3) indicate that two pathw ays are available: one depends on initial dissociation of L and procee ds 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 no t. The dissociative mechanism is predominant for M = Ru, L = PMe3, whe reas the nondissociative or direct mechanism plays the dominant, possi bly 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), w hereas the corresponding PMe2Ph derivative 9 is in equilibrium with ex o-3 and PMe2Ph and cannot be isolated; the ruthenium(0) compound exo-1 is inert toward PMe3. Density functional calculations on the model co mpounds ML3-{eta(4)-exo-o-(CH2)(2)C6H4} and ML4{kappa(2)-o-(CH2)(2)C6H 4}(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 co mputational simplification of replacing a tertiary phosphine by PH3 is probably unjustified, The molecular structures of the eta(4) complexe s endo-3, exo-3, and exo-1 and of the diyl complex 8 have been determi ned by X-ray crystallography. The endo- to exo-o-xylylene isomerizatio ns are compared with the intramolecular migrations that occur in Fe(CO )(3)(eta(4)-polyene) and Cr(CO)(3)(eta(6)-substituted-naphthalene) com plexes.