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The catalytic double-bond shift isomerization of α-olefins by dithio-β-diketonate palladium complexes Pd{CH3C(S)CHC(S)CH3}(PL1L2L3)X(1) and [Pd{CH3C(S)CHC(S)CH3}{Ph2PCH2CH2PPh2}]Y(2)(X = 1, Br; Y = halide, PF6, BPh4) is described. Catalysts are formed on activation of the complexes by a variety of alkylaluminium cocatalysts . Several of the catalysts are highly active for isomerization of oct-1-ene and but-1-ene, activities being higher than normally obtained for palladium systems. In contrast to the majority of palladium-based catalysts product distributions rapidly approached thermodynamic equilibrium, with a strong preference for trans products at each stage of the reaction. Products from the isomerization of oct-1-ene were analysed by g.l.c.-F.t.i.r . in conjunction with Kovats retention indices.

The perfluorinated ion-exchange polymer, Nafion-H+, has been used as a Bronsted acid to activate [Ni{P(OEt)3}4] and as a support for the resulting cationic species. Spectroscopic evidence has confirmed that the species present within the Nafion-H+ film are the same as those found in a homogeneous solution using H2SO4 as the Bronsted acid. Activity for oct-1-ene isomerization in the first 30 min was approximately one third of the activity obtained in homogeneous solution using H2SO4 as co-catalyst with a similar [Ni{P(OEt)3}4]: H+ ratio, and the activity varied linearly with the amount of Nafion-H+ present. Comparative tests with but-1-ene employing homogeneous and supported catalyst systems have demonstrated a significant increase in isomerization rate on changing from oct-1-ene to but-1-ene when using the supported catalyst. This indicates that diffusion of oct-1-ene into, and/or products out of, Nafion-H+ are controlling factors. Diffusion coefficients of oct-1-ene and but-1-ene in Nafion have been determined and support this conclusion. It has also been demonstrated that very little, if any, of the catalyst species is leached from the Nafion support during catalysis.

The complexes [Ni{R1C(S)CR2C(S)R3}2]1 and [Ni{R1C(S)CR2C(S)R3}(PL1L2L3)X]2(R, L = alkyl, aryl or CF3; X = Cl, Br or I) have been prepared. The complexes 2 can be chemically anchored to polystyrene or silica–alumina supports via pendant phosphine ligands to form the derivatives 3. The species 2 and 3 in combination with a suitable co-catalyst form extremely active catalysts for the oligomerization of ethylene, propylene or butenes. The immobilised systems show no significant signs of leaching after 24 h operation. In general the oligomerization is highly selective to dimers, however, selectivity and catalyst activity are markedly affected by changes in L. Changes in R affect catalyst activity only.

The application of non-phosphine-based chelating ligands in homogeneous catalyst systems is a surprisingly recent and relatively unexplored area of research. Chelating ligands can concurrently stabilize intermediates, control catalyst activity and direct the product distribution far more effectively than monodentate ligands. In this review our studies with catalyst systems containing chelate ligands primarily of the P-diketonate type [dithio-P-diketonate (sacsac); monothio-P-diketonate (sacac); and imino P-diketonate (nacac) ligands] is discussed. Examples of the catalyst systems show enzyme-like superactivity. Studies modelling these catalyst systems have provided valuable information relating the effects of ligand modifications to reaction pathways and to activities. Our most recent investigations of simple chelating ligands based on picolinic acid are also discussed. Studies modelling CO/ethene insertion/elimination with extremely labile alkylplatinum picolinate complexes led to the development of new single-component nickel oligomerization catalysts.

Numerous unsaturated substrate molecules undergo migratory insertion with metal-carbon bonds. Recent attention has been focused on insertion of CO and alkenes into d 8 metal hydrocarbyl and acyl bonds with the aim of understanding the fundamental steps involved. Knowledge of ligand control over rates and regioselectivity of the insertion reaction would allow the design of highly active and selective catalyst systems for a whole range of carbon-carbon bond forming reactions. Herein are summarised recent advances in delineating the detailed aspects of the insertion mechanism and ligand influences on rates and regioselectivity. Investigations have provided information on the nature of pre- and post-insertion intermediates; the nature of the carbon-carbon bond forming step; ligand control, and advances in theoretical modelling of the details of the intricate insertion reaction.

A variety of methylpalladium carbene complexes of the type [Pd(Me)(carbene)(chelate)] [where carbene=1,3-di-methylimidazol-2-ylidene (dmiy), chelate=acetylacetonate (acac) 1, trifluoroacetylacetonate (tfac) 2, hexafluoroacetylacetonate (hfac) 3; carbene=1,3,4,5-tetramethylimidazol-2-ylidene (tmiy), chelate=acac, 6] have been synthesised and fully characterised. The bis-carbene complexes trans-[Pd(Me)(carbene)2Cl] [carbene=dmiy, 7 and tmiy, 8] have also been isolated. The unusual cationic complex {[Pd(Me)(dmiy)(cod)]+BF4−} 4, in which there are three distinct Pd–carbon bonds, including cis alkyl/alkene coordination has been prepared and aspects of its chemistry investigated. The complex which slowly decomposes even at −20°C does not react via the expected migratory insertion of the methyl group with the coordinated cod but decomposes via a pathway involving the methyl group and the carbene ligand. Crystal structures of the bis-carbene complex 7 and the chloro-bridged dimer [Pd(Me)(tmiy)Cl]2 5 reveal square planar coordination. The carbene C3N2 arrays are closely planar, with the ligand planes inclined at significant angles to the coordination planes of the respective complexes. For complex 7 the interplanar dihedral angle is 70.4(3)° and for 5 the angle is 65.07(8)°. Examples of the methylpalladium monocarbene and dicarbene complexes were tested in the Heck reaction and were found to be extremely active with several having turnover frequencies (TOF’s) of >20 000 and total turnover numbers (TON’s) of >100 000.

N-Heterocyclic carbenes (NHCs) are commonly regarded as strong donor ligands that are valuable in coordination chemistry and catalysis. Many reports describing this aspect of their chemistry have been published. The alternative view of NHCs as reaction intermediates has been little explored, and yet excellent examples exist and will be reviewed in this perspective. Group 10 hydrocarbyl complexes of NHCs [(R)(NHC)ML2 where R = H, alkyl, aryl, acyl; M = Ni, Pd, Pt] undergo a facile reductive elimination to generate M(0) and R-substituted-azolium salt as products. On the other hand, 2-Hazolium salts will oxidatively add to M(0) complexes to afford reactive NHC–M–H compounds, suitable as catalysts for selected reactions. Combining the oxidative addition and reductive elimination steps into a single cycle, in the presence of an alkene, provides of a novel and potentially exciting, atom efficient catalytic C–C bond forming process for the substitution, and annulation of heterocyclic rings.