Complexes of Halogens

Like palladium(II) and platinum(II), gold(III) has the d8 electronic configuration and is, therefore, expected to form square planar complexes. The d-orbital sequence for complexes like AuC14 is d dxy d7Z, 

The stability of gold(III) compared with silver(III) has been ascribed to relativistic effects causing destabilization of the 5d shell, where the electrons are less tightly held. Hartree-Fock calculations on AuXJ (X = F, Cl, Br) indicate that relativistic effects make a difference of 100-200 kJ mol-1 in favour of the stability of AuXJ (Table 4.12) [110]. 

The oxidizing agent is usually concentrated HN03 but can be the halogen itself yellow fluoroaurates can be made directly or by substitution 

The black iodide is unstable [3(d), 112], tending to reduce to Aul2 in aqueous solution, but has been made in situ 

Apart from Au(N03)4, relatively few complexes of gold(III), and only those with O-donors, have been examined. Two that demonstrate the preference of gold(III) for square planar coordination are SrAu2(MeCOO)8 and SrAu2(OH)8 in the latter Au(OH)4 has Au—O 1.98 A [117]. 

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In understanding these reactions, it is helpful to view the metal-alkene tt complex as an incipient carbocation (just as tt complexes of halogens are incipient carbocations). Alkyl and hydride shifts then bear analogy to carbocation rearrangements. This may be an oversimplification but it makes the chemistry easier to follow. 

The structure of ra-complexes has been examined by the X-ray crystallographic studies of solid samples and by the spectral measurement of heterocycle-halogen equilibria in solution. By the former approach the structure of the solid 1 2 pyridine-molecular iodine complex has been shown to consist principally of two pyridine molecules collinearly bonded to an iodine atom and of linear triiodide units. Moreover, in the 1 1 complex of dioxane and bromine, the heterocyclic oxygen-bromine-bromine linkages are also collinear.4 In the latter type of study, interest in the well-known phenomenon of brown iodine solutions has occasioned the measurement of the stability constants for many complexes of halogen and heterocycles.25, 29 Information concerning their structure in solution comes from a consideration of the relative size of such constants as a function of heterocycle structure. Thus, the fact that bromine complexes of both 8-bromo- and 8-methylquinolines possess stability constants (K = 1.1 and 4.8 liters/mole) much smaller than that of quinoline itself... 

Alternatively, as in Scheme B, BrCN forms only an outer sphere complex, with Fe(HjO) assuming the role of an acid catalyst. In both schemes the essential feature is a polarization of the YH-BrCN system in which YH acts as a donor and BrCN as an acceptor, not unlike the situation in molecular complexes of halogens with [Pg.384]

From a mechanistic point of view, two different ionic mechanisms have to be considered (due to the presence of oxygen the radical chain mechanism plays no role in the technical process) first, the uncatalyzed reaction of ethylene and chlorine and second, the metal halide catalyzed reaction. Both routes compete in this process. The uncatalyzed halogenation was studied extensively for the bromina-tion of olefins [14, 15] (Scheme 4). It is commonly accepted that the halogenation of olefins starts with formation of a 1 1 -complex of halogen and alkene followed by formation of a bromonium ion. Subsequent nucleophilic attack of a bromine anion leads to the dibromoalkane. However, when highly hindered olefins (such as tetraneopentylethylene) are used, formation of a 2 1 r-complex, as an intermediate between 1 1 ir-complex and a bromonium ion, is detectable by UV spectroscopy. In the catalyzed reaction the metal halide polarizes the chlorine bond, thus leading to formation of a chloronium or carbonium ion. Subsequent nucleophilic attack of a chloride anion gives the dichloroalkane [12] (Scheme 5). 

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