Aryl complexes metal- 7 bonds

Metal formyl complexes have been proposed as important intermediates in the metal-catalyzed reduction of CO by H2 1,2, 3, 4). While the insertion of CO into alkyl and aryl carbon-metal bonds is well known (5), the insertion of CO into a metal-hydrogen bond to give a metal formyl complex has not been observed. (The intermediacy of metal formyl compounds in the substitution reactions of metal hydrides has been considered.) To ascertain the reasons for the failure to observe metal formyl complexes in the reactions of metal hydrides with CO, we have developed a new synthesis of metal formyl complexes and have studied their properties. 

Many Cu(I) compounds have polymeric structures with weak Cu—Cu bonds that are bridged by atoms or groups. These include Cu(I) carboxylates, alkyls and aryls, alkoxides and (CuXL) complexes (X = halide, L = ligand). In Cu(I) compounds Cu has a filled 3d shell, 3d , that does not participate in metal-metal bonding, so the extent of metal-metal bonding in these compounds is questionable. Calculations show that the metal-metal bonding is at best weak . These compounds arise from the same syntheses as would be used to prepare the monomer, and so they are not considered further here. 

Examples of silver(l) alkyl and alkenyl (including aryl) complexes have been known from as early as 1941 6-9 however, the number of examples is fairly limited with respect to that of the heavier congeners, copper(l) and gold(l). Such a phenomenon can readily be attributed to the relatively low stability of this class of complexes, both photochemically and thermally. Simple homoleptic alkyl and alkenyl complexes of silver(i) are known to be very unstable under ambient temperature and light, and successful isolation of this class is fairly limited and mainly confined to those involving perfluoroorganics.10 The structures and the metal-carbon bond-dissociation energies for... 

The catalytic cycles for reduction of alkyl and atyl halides using Ni(o), Co(i) or Pd(o) species are interrupted by added carbon dioxide and reaction between the first formed carbon-metal bond and carbon dioxide yields an alkyl or aryl car-boxylate. These catalyses reactions have the advantage of occuriiig at lower cathode potentials than the direct processes summarised in Table 4.14. Mechanisms for the Ni(o) [240] and Pd(o) [241] catalysed processes have been established. Carbon dioxide inserts into the carbon-metal bond in an intermediate. Once the carboxy-late-metal species is formed, a further electron transfer step liberates the carboxy-late ion reforming the metallic complex catalyst. 

An interesting compound is [NiBr(Pr2CH-DAB)]2 which formally contains Ni(I). Dimerization leads to a diamagnetic complex with a metal-metal bond. The compound may be prepared by various routes which may involve (i) reduction of NiBr2(R-DAB) with Na, (ii) reaction of Ni(R-DAB)2 with NiBr2 (R-DAB), (iii) treatment of Ni(R-DAB)2 with R Br and finally (iv) reaction of Ni(cod)(R-DAB) with aryl bromide.147-149... 

Sulfur dioxide is capable of reacting with metal alkyl, aryl or u-allyl complexes in an insertion-type reaction to yield S-sulfinate (4), O-sulfinate (5) or 0,< -sulfinate complexes (6).13 It can also insert into the metal-metal bond in the cobalt complex (7) to give the S02-bridged complex (8).38... 

Most of the reactions listed in Table 6 involve prior activation of the substrate by coordination to palladium in the form of a v-, a 77-ally lie, a 77-benzylic, or an alkyl or aryl complex. Once coordinated to the metal, the substrate becomes an electron acceptor and can react with a variety of different nucleophiles. The addition of nucleophiles (Nu) to the coordinated substrate may occur in two different ways, as shown by Scheme 9 for 7r-alkene complexes 397"399 (a) external attack leading to trans addition of palladium and nucleophile across the 77-system (path A) or (b) internal addition of the coordinated nucleophile to the complexed alkene resulting in cis addition of palladium and nucleophile to the double bond. The cis and trans adducts (120) and (121) may then undergo /3-hydride elimination (/3-H), producing the vinylic oxidation product... 

The reactions shown in Scheme 1 require activation of the aromatic C-H bond by a metal and subsequent insertion of an alkene or alkyne in the aryl-carbon palladium bond (Chapter III.1.3.2.5). C-H activation has been the topic of many studies since the 1960s and several metal complex systems are known to induce... 

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