Reversible CO Insertion into an Ni–C Bond of Alkyl(acetylacetonato)(triphenylphosphine)nickel(II)

Reaction of alkyl(acetylacetonato)(triphenylphosphine)nickel(II), RNi(acac)(PPh3)n (R=C2H5, n=1 (1a); R=CH3, n=2 (1b)) with carbon monoxide gives acyl(acetylacetonato)(triphenylphosphine)nickel(II) RCONi(acac) (PPh3) (R=C2H5 (2a); R=CH3 (2b)). Elemental analysis, IR and NMR spectra, and chemical properties of the acylnickel complexes support the structure. A variable temperature NMR spectroscopic study has revealed that the acac ligand in the acyl complexes exchanges rapidly in solution on NMR time scale. The acylnickel complexes 2a and 2b tend to decarbonylate at room temperature, the decarbonylation tendency of 2b being greater than 2a. They undergo further disproportionation according to the stoichiometry: 3RCONi(acac)(PPh3)→RCOR+Ni(CO)2(PPh3)2+Ni(acac)2+RNi(acac)(PPh3). The mechanism involving the cross coupling of the acylnickel complex with the alkylnickel complex is proposed for the reaction on the basis of formation of CH3COC2H5 on reaction of 2a with 1b.