Transition metal complexes Nickel

Organic electroreductive coupling reactions using transition-metal complexes as catalysts have been widely investigated. Reviews on the subject have been published [89, 90]. The method involving the most common transition-metal complexes (nickel, cobalt, palladium) appears to be a useful tool to synthetize heterocycles from organic halides via radical intermediates. Nickel catalyst precursors are nickel(II) salts that are cathodically reduced either to nickel(I) or to nickel(O) and cobalt catalyst... 

Transition metal atoms may also form complexes via dative hond formation. The extraction of nickel involves the formation and thermal decomposition of a transition metal complex. Nickel is usually extracted from its sulfide ore. This is roasted in air to give nickel(ii) oxide, NiO, which is then reduced by carbon to the impure metal. Refinement (purification) can be achieved by reaction with carbon monoxide at about 50 °C and normal pressure, which results in the formation of tetracarbonylnickel(O), NifCO). This complex is easily thermally decomposed to form extremely pure nickel (Mond process). 

Hydrosilanes undergo addition to carbon-carbon multiple bonds under catalysis by transition metal complexes. Nickel, rhodium, palladium, and platinum were used as catalytically active metals. By incorporating chiral ligands into the metal catalyst, the hydrosilylation can be performed analogously to other addition reactions with double bonds, for example, asymmetric hydrogenation to obtain optically active alkylsilanes. 

A large number of organometallic compounds are based on transition metals Examples include organic derivatives of iron nickel chromium platinum and rhodium Many important industrial processes are catalyzed by transition metals or their complexes Before we look at these processes a few words about the structures of transition metal complexes are m order... 

With an atomic number of 28 nickel has the electron conflguration [Ar]4s 3c (ten valence electrons) The 18 electron rule is satisfied by adding to these ten the eight elec Irons from four carbon monoxide ligands A useful point to remember about the 18 electron rule when we discuss some reactions of transition metal complexes is that if the number is less than 18 the metal is considered coordinatively unsaturated and can accept additional ligands... 

Metal Complex. Complexation gas chromatography was first introduced by V. Schurig in 1980 (118) and employs transition metals (eg, nickel, cobalt, manganese or rhodium) complexed with chiral terpenoid ketoenolate ligands such as 3-ttifluoroacetyl-lR-camphorate (6),... 

Although trialkyl- and triarylbismuthines are much weaker donors than the corresponding phosphoms, arsenic, and antimony compounds, they have nevertheless been employed to a considerable extent as ligands in transition metal complexes. The metals coordinated to the bismuth in these complexes include chromium (72—77), cobalt (78,79), iridium (80), iron (77,81,82), manganese (83,84), molybdenum (72,75—77,85—89), nickel (75,79,90,91), niobium (92), rhodium (93,94), silver (95—97), tungsten (72,75—77,87,89), uranium (98), and vanadium (99). The coordination compounds formed from tertiary bismuthines are less stable than those formed from tertiary phosphines, arsines, or stibines. 

In the nitrone cycloaddition reactions catalyzed by the l ,J -DBFOX/Ph transition metal complexes also, the diastereo- and enantioselectivities were found to depend upon the presence of MS 4 A [71]. Thus, both the selectivities were much lowered in the iron(II) or nickel(II) complex-catalyzed reactions without MS 4 A,... 

Monodentate dipolarophiles such as acrolein, methacrolein, and a-bromoacrolein could be successfully utilized in the l ,J -DBFOX/Ph-transition metal complex-catalyzed asymmetric nitrone cycloadditions [76]. The reactions of N-benzylideneani-line N-oxide with acrolein in the presence of the nickel(II) aqua complex R,R-DBF0X/Ph-Ni(C104)2 3H20 (10mol%) and MS 4 A produced a mixture of two regioisomers (5-formyl/4-formyl regioisomers ca 3 1). However, enantio-... 

Coordination-catalyzed ethylene oligomerization into n-a-olefins. The synthesis of homologous, even-numbered, linear a-olefins can also be performed by oligomerization of ethylene with the aid of homogeneous transition metal complex catalysts [26]. Such a soluble complex catalyst is formed by reaction of, say, a zero-valent nickel compound with a tertiary phosphine ligand. A typical Ni catalyst for the ethylene oligomerization is manufactured from cyclo-octadienyl nickel(O) and diphenylphosphinoacetic ester ... 

Among many examples of -orbital interaction, only the following two are selected to illustrate the feature of HO—LU conjugation. One is the cyclooctadiene-transition metal complex ">. The figure indicates the symmetry-favourable mode of interaction in a nickel complex. The electron configuration of nickel is (3d)8 (4s)2. The HO and LU of nickel can be provided from the partly occupied 3d shell from which symmetry-allowed occupied and unoccupied d orbitals for interaction with cyclo-octadiene orbitals are picked up. 

The first examples of highly active olefin polymerization catalysts based on late transition metals were nickel and palladium complexes containing bulky diimine ligands.310 312 For example, complex (120) was found to polymerize ethylene with an activity of ll,000gmmol h bar A range of PE materials with molecular weights up to 106 and... 

The catalytic cyclo-oligomerization of 1,3-butadiene mediated by transition-metal complexes is one of the key reactions in homogeneous catalysis.1 Several transition metal complexes and Ziegler-Natta catalyst systems have been established that actively catalyze the stereoselective cyclooligomerization of 1,3-dienes.2 Nickel complexes, in particular, have been demonstrated to be the most versatile catalysts.3... 

Dimerization, oligomerization, and similar reactions of olefins have been reported to be catalyzed by systems involving the majority of the Group VIII metals (3). The reasons for the particular interest in nickel-containing catalysts are their exceptionally high catalytic activity (catalytic reactions have been performed at temperatures as low as - 100°C), the diversity of catalytic reactions of obvious synthetic value, as well as the possibility to direct the course and control the selectivity of a catalytic reaction by tailoring the catalyst which are perhaps without parallel among transition metal complex catalysts. 

Among transition metal complexes used as catalysts for reactions of the above-mentioned types b and c, the most versatile are nickel complexes. The characteristic reactions of butadiene catalyzed by nickel complexes are cyclizations. Formations of 1,5-cyclooctadiene (COD) (1) and 1,5,9-cyclododecatriene (CDT) (2) are typical reactions (2-9). In addition, other cyclic compounds (3-6) shown below are formed by nickel catalysts. Considerable selectivity to form one of these cyclic oligomers as a main product by modification of the catalytic species with different phosphine or phosphite as ligands has been observed (3, 4). 

It should also be noted that this polymerization system is not disturbed in the presence of alcohol and water. Similar polymerizations with nickel [278,279] and iron [280] complexes have also been reported. The structures of the transition metal complexes are shown ... 

The reductive cyclization of non-conjugated diynes is readily accomplished by treatment of the acetylenic substrate with stoichiometric amounts of low-valent titanium52 523 and zirconium complexes.53 533 Hence, it is interesting to note that while early transition metal complexes figure prominently as mediators of diyne reductive cyclization, to date, all catalyzed variants of this transformation employ late transition metal complexes based on nickel, palladium, platinum, and rhodium. Nevertheless, catalytic diyne reductive cyclization has received considerable attention and is a topic featured in several review articles. ... 

In homogeneous solutions NO disproportionation maybe promoted by transition metal complexes, and a variety of mechanisms seem to be available owing to the many possible modes of coordination. One example is the reaction of NO with nickel carbonyl shown in Eq. (35) (76),... 

Many aquatic organisms exhibit an ability to concentrate a variety of trace elements and this ability has been identified as a function of the tendency of the elements to be complexed by ligands (159). The alkaline earth metals are poorly com-plexed in relation to the transition metals, copper, nickel, cobalt, zinc and manganese. The actinides should be regarded as members of an intermediate group. It has been suggested by Martin (160) that at least five mechanisms may regulate the uptake of metals by marine biota. These are... 

In this context, homogeneous catalysis has been increasingly used since it can offer valuable advantages, notably in terms of selectivity and efficiency. Indeed, low-valent complexes of transition metals like nickel, palladium, or cobalt can react with many functionalities, thus allowing numerous C,C-bond forming reactions. 

As a guide for going through this review it can be helpful to consider briefly the electrochemical and chemical behavior of the most common transition metal complexes i.e., nickel, palladium and cobalt, used in the various reactions collected here. Additional data can be found in [1]. 

Organic Electroreductive Coupling Reactions using Transition Metal Complexes as Catalysts Table 2. Reductive electropolymerisation of aryl dihalides using nickel catalysts... 

---