Halide Abstraction Competes with Oxidative Addition in the Reactions of Aryl Halides with [Ni(PMen Ph(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(PMen Ph(3-n) )4 ], revealing the crucial role of an open-shell singlet transition state for halide abstraction. The formation of NiI versus NiII has been rationalised through the study of three different pathways: (i) halide abstraction by [Ni(PMen Ph(3-n) )3 ], via an open-shell singlet transition state; (ii) SN 2-type oxidative addition to [Ni(PMen Ph(3-n) )3 ], followed by phosphine dissociation; and (iii) oxidative addition to [Ni(PMen Ph(3-n) )2 ]. For the overall reaction between [Ni(PMe3 )4 ], PhCl, and PhI, a microkinetic model was used to show that our results are consistent with the experimentally observed ratios of NiI and NiII when the PEt3 complex is used. Importantly, [Ni(PMen Ph(3-n) )2 ] complexes often have little, if any, role in oxidative addition reactions 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.