Synthesis of [Ni(g 2 -CH 2 C 6 H 4 R-4){PPh(CH 2 CH 2 PPh 2 ) 2 }] + (R = H, Me or MeO) and protonation reactions with HCl Valerie Autissier, Ellie Brockman, William Clegg, Ross W. Harrington, Richard A. Henderson * Department of Chemistry, School of Natural Sciences, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK Received 7 December 2004; accepted 14 January 2005 Available online 8 March 2005 Abstract The complexes [Ni(g 2 -CH 2 C 6 H 4 R-4)(triphos)]BPh 4 {R = H, Me or MeO; triphos = PhP(CH 2 CH 2 PPh 2 ) 2 } have been prepared and characterised by spectroscopy and X-ray crystallography. In all cases the coordination geometry of the nickel is best described as square-planar with an g 2 -benzyl ligand occupying one of the positions. The orientation of the g 2 -benzyl ligand is dictated by the steric restrictions imposed by the phenyl groups on the triphos ligand, so that the phenyl group on the unique secondary phosphorus and the aromatic group of the benzyl ligand (which are trans to one another) are oriented in the same direction. [Ni(g 2 -CH 2 C 6 H 4 R- 4)(triphos)] + react with an excess of anhydrous HCl in MeCN to form [NiCl(triphos)] + (characterised as the [BPh 4 ]  salt by X-ray crystallography) and the corresponding substituted toluene. The kinetics of the reaction of all [Ni(g 2 -CH 2 C 6 H 4 R-4)(triphos)] + and HCl in the presence of Cl  have been determined using stopped-flow spectrophotometry. All reactions exhibit a first-order depen- dence on the concentration of complex and a first-order dependence on the ratio [HCl]/[Cl  ]. Varying the 4-R-substituent on the benzyl ligand shows that electron-withdrawing substituents facilitate the rate of the reaction. It is proposed that the mechanism involves initial rapid protonation at the nickel to form [NiH(g 2 -CH 2 C 6 H 4 R-4)(triphos)] 2+ , followed by intramolecular proton migration from nickel to carbon to yield the products. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Nickel; Benzyl; Protonation; Mechanism 1. Introduction Studies on the kinetics of protonation of transition metal complexes are of fundamental interest in under- standing the mechanisms of proton transfer to inorganic complexes as well as being relevant to the action of cer- tain industrially important catalysis, such as hydrocya- nation [1]. Whilst there have been significant advances in understanding the mechanisms of protonation of transition metal complexes, the following aspects are still unresolved [2]. (i) The electronic and steric factors which control the rate of protonation of metal com- plexes. Understanding these factors will allow us to predict the sites of protonation in new reactions. (ii) Elucidation of the kinetically favoured site of pro- tonation of the complex, in particular, investigating whether a proton binds directly to the site identified in the product, or whether the proton binds initially to some other site (kinetically controlled product) and then moves to the ultimate site (thermodynamically con- trolled product). (iii) Probing the pathways by which protons move between the kinetically favoured and the thermodynamically favoured sites. These pathways can be intramolecular or intermolecular. Earlier work from our laboratories focussed on the protonation of complexes based on the substitutionally robust {M(Ph 2 PCH 2 CH 2 PPh 2 ) 2 } (M = Mo or W) core, 0022-328X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2005.01.034 * Corresponding author. Tel.: +44 191 222 6636; fax: +44 191 222 6929. E-mail address: r.a.henderson@ncl.ac.nk (R.A. Henderson). Journal of Organometallic Chemistry 690 (2005) 1763–1771 www.elsevier.com/locate/jorganchem