Halide abstraction competes with oxidative addition in the reactions of aryl halides with [Ni(PMe_nPh(_3-n))₄]

Density functional theory (DFT) calculations have been used to study the oxidative addition of aryl halides to complexes of the type [Ni(PMe_nPh_(3-n))₄], revealing the crucial role of an open shell singlet transition state for halide abstraction. The formation of NiI versus Ni^II has been rationalised through the study of three different pathways: (i) halide abstraction by [Ni(PMe_nPh_(3-n))₃], via an open shell singlet transition state; (ii) S_N2-type oxidative addition to [Ni(PMe_nPh_(3-n))₃], followed by phosphine dissociation; and (iii) oxidative addition to [Ni(PMe_nPh_(3-n))₂]. For the case of [Ni(PMe₃)₄], a microkinetic model was used to show that these data are consistent with the experimentally-observed ratios of NiI and Ni^II. Importantly, [Ni(PMe_nPh_(3-n))₂] complexes often have little if any role in the oxidative addition reaction because they are relatively high in energy. The behaviour of [Ni(PR₃)₄] complexes in catalysis is therefore likely to differ considerably from those based on diphosphine ligands in which two coordinate Ni0 complexes are the key species undergoing oxidative addition.