Linear species group 2 halides

Secondary bromides and tosylates react with inversion of stereochemistry, as in the classical SN2 substitution reaction.24 Alkyl iodides, however, lead to racemized product. Aryl and alkenyl halides are reactive, even though the direct displacement mechanism is not feasible. For these halides, the overall mechanism probably consists of two steps an oxidative addition to the metal, after which the oxidation state of the copper is +3, followed by combination of two of the groups from the copper. This process, which is very common for transition metal intermediates, is called reductive elimination. The [R 2Cu] species is linear and the oxidative addition takes place perpendicular to this moiety, generating a T-shaped structure. The reductive elimination occurs between adjacent R and R groups, accounting for the absence of R — R coupling product. 

The methyl arsenic system compromises two well-defined species, the pentamer (MeAs)5, a yellow pyrophoric oil (m.p. 12°C) and the purple black double-chain polymer, [(MeAs)2] (m.p. 204 °C). A red solid that is possibly the linear chain compound (MeAs) has also been described. The polymers are formed when samples of the pentamer are exposed to impurities like arsenic halides that are able to react with the arsenic-arsenic bond and may act as end groups. The most widely used method for the preparation of larger amounts of (MeAs)j is the reduction of methylarsonic acid, MeAsO(OH)2, or its sodium salt with hypophosphoric acid " °. Samples prepared by this method, however, may contain between 5 and 15% of impurities. A method that minimizes impurities is the reaction of MeAsHj with dibenzylmercury (equation 3) °. 

Two groups have recently claimed to have prepared the radical H2S", which has two electrons more than NH2 or PH2 (14, 56). One group prepared their radical by photolysis of alkali halide crystals doped with HS" ions. At 20°K. only trapped hydrogen atoms were detected, but after annealing at 110°K. for a few seconds and recooling, spectra assigned to S and H2S" were obtained. The results for the latter species, summarized in Table VI, show indeed that one sulfur and two equivalent protons are present. Since the protons remain magnetically equivalent for all orientations, the molecule was taken to be linear. 

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