Complexes of Unsaturated Hydrocarbons

Many alkenes and arenes react directly with dissolved silver salts to afford crystals of the silver complex. Examples studied by X-ray diffraction [151] include (C6H6)AgX (X = C104, A1C14) and (C8H8)AgN03. 

The Ag-C bonds tend to be asymmetric study of silver cycloalkene complexes shows their stability to decrease in the order C5 C6 C7 C8, corresponding to relief of strain in the cyclic molecules consequent upon the lengthening of the double bond on coordination. 

There are considerable numbers of the organogold compounds [3(b), 9,154], principally in the +1 and -1-3 oxidation states. Gold is unusual in transition metals in that, even in the H-1 state, it has a marked preference for forming r-rather than 7r-bonds, presumably related to the tendency of gold(I) to linear 2-coordination. 

Current study in this area is prompted by laser-induced CVD of such volatile gold compounds, permitting direct laser writing in gold [155]. 

Simple alkyls and aryls AuR are generally unstable but coordinative saturation ensures the stability of adducts Au(PR3)R  

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III. LITHIUM COMPLEXES OF UNSATURATED HYDROCARBONS A. The Li(C2H2)i,2 and LI2C2H2 Compounds... 

The first metal-olefin complex was reported in 1827 by Zeise, but, until a few years ago, only palladium(II), platinum(Il), copper(I), silver(I), and mercury(II) were known to form such complexes (67, 188) and the nature of the bonding was not satisfactorily explained until 1951. However, recent work has shown that complexes of unsaturated hydrocarbons with metals of the vanadium, chromium, manganese, iron, and cobalt subgroups can be prepared when the metals are stabilized in a low-valent state by ligands such as carbon monoxide and the cyclopentadienyl anion. The wide variety of hydrocarbons which form complexes includes olefins, conjugated and nonconjugated polyolefins, cyclic polyolefins, and acetylenes. 

Polymerisations and copolymerisations of heterounsaturated and heterocyclic monomers in the presence of coordination catalysts constitute a distinct group of coordination polymerisation processes. Considering the nature of the coordination bond of the a type between the monomer heteroatom (beyond carbon monoxide [60]) and the metal atom, the complexes formed differ essentially from the re complexes of unsaturated hydrocarbon monomers with transition metals. 

Temkin and co-workers have investigated the thermodynamic properties of the soluble complexes of unsaturated hydrocarbons with various metal salts with particular reference to their role in catalytic reactions. Using a potentio-metric technique, they were able to calculate the thermodynamic data shown in Table 6 for the silver(I)-acetylene complexes 30) and the silver(I)-ethylene complex 31). The results obtained for acetylene have been related to the low activity of silver salts as catalysts for the hydration of acetylene. For the sil-ver(I)-ethylene complex, the relationship between the ionic concentrations and... 

Various transition metal complexes readily abstract two hydrogen atoms from C-H compounds yielding ju-olefin. derivatives. If the 7t-complexes of unsaturated hydrocarbons formed in the dehydrogenation process are relatively unstable, the Jt-ligand easily dissociates and the reaction becomes catalytic with respect to metal complex ... 

Nucleophilic additions to 18-electron, cationic transition metal complexes of unsaturated hydrocarbons have been particularly well studied. Davies, Green, and Mingos have developed guidelines to predict the most kinetically favorable position for nucleophilic... 

The complexation of unsaturated hydrocarbons by transition metals is a powerful activation method that plays a fundamental role in their stoichiometric and catalytic transformation. The bonds formed are governed by jt backbonding. Olefins, dienes and arenes are usually 2-electron L, 4-electron L2, and 6-electron L3 ligands respectively, and alkynes are 2-electron L or 4-electron L2 depending on the needs of the metal. The odd-electron ligands are the 3-electron LX allyl radical and 5-electron L2X dienyl radicals. Cyclopentadienyl and arene sandwich complexes will be examined in Chap. 11. 

The complex [Fe(CO)3(ri" -13-cyclohexadiene)], useful in organic synthesis, is obtained by complexation-isomerization of the 1,4-isomer by reaction with [Fe(CO)5]. There are also other robust 18-electron complexes of unsaturated hydrocarbons as L2 ligands that are related to dienes but are instable in the free state cyclobutadiene, orthoxylylene (orthoquinodimethane) and trimethylenemethane (below). 

A broad scope is documented for the preparation of suspensions of 2-alkenylpotassium lithium toV-butoxide complexes from unsaturated hydrocarbons by means of butyllithium/ potassium /erf-butoxidc (Schlosser Lochmann base LICKOR reagents )38-45,432 456. Examples are given in Section D.1.3.3.3.3.2.1. 

A much more detailed and time-dependent study of complex hydrocarbon and carbon cluster formation has been prepared by Bettens and Herbst,83 84 who considered the detailed growth of unsaturated hydrocarbons and clusters via ion-molecule and neutral-neutral processes under the conditions of both dense and diffuse interstellar clouds. In order to include molecules up to 64 carbon atoms in size, these authors increased the size of their gas-phase model to include approximately 10,000reactions. The products of many of the unstudied reactions have been estimated via simplified statistical (RRKM) calculations coupled with ab initio and semiempirical energy calculations. The simplified RRKM approach posits a transition state between complex and products even when no obvious potential barrier... 

In most palladium-catalyzed oxidations of unsaturated hydrocarbons the reaction begins with a coordination of the double bond to palladium(II). In such palladium(II) olefin complexes (1), which are square planar d8 complexes, the double bond is activated towards further reactions, in particular towards nucleophilic attack. A fairly strong interaction between a vacant orbital on palladium and the filled --orbital on the alkene, together with only a weak interaction between a filled metal d-orbital and the olefin ji -orbital (back donation), leads to an electrophilic activation of the alkene9. 

Therefore, the measurement of the relative reactivities in separate and in competitive experiments will permit the evaluation of either K jK or KiK IK K depending upon whether the principal surface species are the TT-complexed multiply unsaturated hydrocarbons or the respective half-hydrogenated states. If the former situation exists, the evaluated ratios might be expected to correlate with the association constants of the hydrocarbons with silver ion (78), but not if the main surface species are the half-hydrogenated states. Apparently, it is the latter condition which prevails. 

Alkanes can be activated by both hard and soft transition metal complexes. Hard catalysts have been known for a long time, although examples involving alkanes are few in number, and all are from studies of recent years. Soft catalysts have become important and have been much studied during the past decade and are widely used for reactions of unsaturated hydrocarbons. It was the use of such soft catalysts that Halpem had particularly in mind when he spoke of the development to activate alkanes as an important and challenging problem. 

An early and popular complex, [IrCl(CO)(PPh3)2], called the Vaska complex,113 and its derivatives can be used for the hydrogenation of unsaturated hydrocarbons.26,31... 

Bond [294] used comparisons between homogeneously and heterogeneously catalysed interconversions of unsaturated hydrocarbons to deduce that the reactive state of an adsorbed hydrocarbon may reasonably be assumed to be a jr-complex (see Sect. 3.2, p. 22). On this assumption, a molecular orbital model appropriate to a face-centred cubic metal was developed. By considering the direction of emergence and degree of occupation of the metal atomic orbitals at the (100), (110) and (111) faces, assuming that the atomic orbitals on the surface keep the same orientation as in the bulk metal, which may not be valid [295], he concluded that the (111) planes were least suited to the adsorption requirements of... 

Silver ions form similar alkene complexes which are soluble in aqueous solution and may be used to effect the separation of unsaturated hydrocarbons from alkanes. Catalysis for the polymerization of alkenes also form metal-alkene complexes which lead to polymerized product. 

Reductions. Hydrogenation of Mono- and Polyolefins and Acetylenes. Catalysis of hydrogenation of unsaturated hydrocarbons has been studied widely. Catalysis by a variety of metal complexes including those... 

Recently, it was found that the triangular skeleton can be easily destroyed in reactions which most likely include an electron transfer step. For example, the reduction of gem-dichloro[3]triangulane 66 with lithium in f-butanol (which is the standard method for the synthesis of cyclopropanes) unexpectedly gave 50% of unsaturated hydrocarbon (equation 45)88. An even more complex mixture of hydrogenolysis products was obtained in the reduction of dichloride 67. ... 

The complementary approach, activation of unsaturated hydrocarbons toward electrophilic attack by complexation with electron-rich metal fragments, has seen limited investigation. Although there are certainly opportunities in this area which have not been exploited, the electrophilic reactions present a more complex problem relative to nucleophilic addition. For example, consider the nucleophilic versus electrophilic addition to a terminal carbon of a saturated 18-electron metal-diene complex. Nucleophilic addition generates a stable 18-electron saturated ir-allyl complex. In contrast, electrophilic addition at carbon results in removal of two valence electrons from the metal and formation of an unstable ir-allyl unsaturated 16-electron complex (Scheme 1). 

From phosphorus heterocycles to phosphorus analogues of unsaturated hydrocarbon-transition metal 7r-complexes. J. Organomet. Chem. 400, 149 (1990). 

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