Oxidation–reduction exchange of 2-acylenolato and trimethylphosphane ligands between d6 and d8 complexes of nickel and cobalt

A series of five (C:O)-dianionic 1-acyl-2-enolato ligands C(O)CCO in a transmetallation reaction between 18-electron metal-d6 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-d8 compound CoMe(PMe3)4 (1:2), undergo a reductive coupling of acyl and methylnickel functions involving the C(O)CCO ligands. Subsequent migration of methyl 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-ray crystal structure of compound 7 has been determined. The methyl ligands are found in positions opposite to the O-donor functions. Upon replacing the aromatic rings in salicylaldehyde derivatives C(O)CCO by more flexible cyclohexene rings or acyclic systems the reduction of Ni(IV) complexes follows a different course.