Ligand displacement reactions. Part V. The mechanisms of displacement of bidentate olefins from olefin(tetramethyl-1,4-benzoquinone)-nickel(0) complexes by trimethyl phosphite

Abstract

Tributylphosphine reacts with the complexes olefin(tetramethyl-1,4-benzoquinone)nickel (olefin = cyclooctatetraene or cyclo-octa-1,5-diene) to give bis(tributylphosphine)(tetramethyl-1,4-benzoquinone)nickel. With trimethyl phosphite the olefin complexes give tetrakis(trimethyl phosphite)nickel and, although intermediates cannot be detected, it is shown that displacement of the olefin precedes that of the quinone. The rate laws found for the reactions of trimethyl phosphite (P) with various olefin complexes (C) are for cyclo-octa-1,5-diene, rate ={k₁[C][P]+k₂[C][P]²}/(1 +k₃[P]); for cyclo-octatetraene and norborna-2,5-diene, rate =k₁[C][P]+k₂[C][P]²; for endo-dicyclopentadiene rate =k₁[C][P]. The reactions of trimethyl phosphite with bis(tetramethyl-1,4-benzoquinone)nickel and of tributylphosphine with the cyclo-octa-1,5-diene complex have rate =k₁[C][P]. The results are accommodated by a mechanism which involves initial bimolecular reaction of (C) and (P) to form an intermediate (A) in which the olefin is monodentate. Intermediate (A) can undergo unimolecular reversion to (C) and (P), unimolecular loss of olefin, or bimolecular reaction with (P) with loss of olefin. Subsequent reactions with trimethyl phosphite are fast. Stationary state theory applied to [A] gives a rate equation of the same form as that found for reaction of the cyclo-octa-1,5-diene complex with trimethyl phosphite. The rate laws for the other complexes are limiting forms of this equation. Primary rate constants are derived and the rate of initial reaction of (P) with (C) is shown to depend on olefin in the order: (tetramethyl-1,4-benzoquinone) cyclo-octa-1,5-diene < endo-dicyclopentadiene < norborna-2,5-diene and cyclo-octatetraene.