Oxidation-reduction exchange of 2-acylenolato and trimethylphosphane ligands between d(6) and d(8) complexes of nickel and cobalt

A series of five (C:O)-dianionic 1-acyl-2-enolato ligands C(O)-C=C-O in a t ransmetallation reaction between 18-electron metal-d(6) complexes mer-trans -NiMe(X)[C(O)-C=C-O](PMe3)(2) [C=C=C6H4, X = I (1); C=C=3-CMe3-5-Me-C6H2, X =I (2); C=C=5,6-benzo-C6H2, X = I (3); C=C=3,4-benzo-cyclohexene, X = Br (4 ); C=C=1,2-diphenylethene, X = I (5)] and the 18-electron metal-d(8) compou nd CoMe(PMe3)(4) (1:2), undergo a reductive coupling of acyl and methylnick el functions involving the C(O)-C=C-O ligands. Subsequent migration of meth yl groups between cobalt intermediates generates three products: mer-trans- CoMe2[MeC(O)-C=C-O](PMe3)(2) (6-10), Ni(PMe3)(4), and CoX(PMe3)(3). The X-r ay crystal structure of compound 7 has been determined. The methyl ligands are found in positions opposite to the O-donor functions. Upon replacing th e aromatic rings in salicylaldehyde derivatives C(O)-C=C-O by more flexible cyclohexene rings or acyclic systems the reduction of Ni(IV) complexes fol lows a different course. (C) 2000 Elsevier Science S.A. All rights reserved .