Ethylene, complex with nickel

Tridentate ligands of the linear type having donor sets P3, AS3 or P As3. t, where the donor atoms are connected by o-phenylene, ethylene or trimethylene chains, were found to form stable complexes with nickel(II) salts.1350 Listings of the most common ligands and nickel(II) complexes are given in Tables 67 and 68, respectively. 

Scheme 3-8 Schematic of ethylene oligomerization with nickel complex catalysts [9]...

Nickel atoms have also been allowed to react with C2H4 under cryogenic conditions (101,123). Depending on the metal-concentration conditions and the deposition temperature, either mononuclear species, Ni(C2H4) , n = 1-3(123), or multinuclear species, Ni2(C2H4) ,m = 1-2, and Ni3(CjH4)i, may be isolated. Unlike the copper complexes, these species are all colorless the mononuclear ethylene complexes each dis-... 

Nickel and palladium react with a number of olefins other than ethylene, to afford a wide range of binary complexes. With styrene (11), Ni atoms react at 77 K to form tris(styrene)Ni(0), a red-brown solid that decomposes at -20 °C. The ability of nickel atoms to coordinate three olefins with a bulky phenyl substituent illustrates that the steric and electronic effects (54,141) responsible for the stability of a tris (planar) coordination are not sufficiently great to preclude formation of a tris complex rather than a bis (olefin) species as the highest-stoichiometry complex. In contrast to the nickel-atom reaction, chromium atoms react (11) with styrene, to form both polystyrene and an intractable material in which chromium is bonded to polystyrene. It would be interesting to ascertain whether such a polymeric material might have any catal3dic activity, in view of the current interest in polymer-sup-ported catalysts (51). 

If this mechanism is correct, the aconitase reaction is an excellent illustration of the influence of the stereochemistry of the metal, as well as its charge, upon the course of a biochemical reaction. The charge on the iron is, of course, responsible for the formation of the resonating carbonium ions A and B from C, D, or E. In C and D the flow of electrons toward iron severs the bond between carbon and the hydroxyl group, whereas in E the proton is released from coordinated water and attached to one of the two ethylenic carbon atoms. The stereochemistry of the iron atom can be credited with holding the organic molecule and the hydroxide in their proper spatial relationship in A and B. It has been recently demonstrated that the complexes of the aconitase substrates with nickel have the structures postulated by Speyer and Dickman and shown in Figure 3 (19). 

There has been some work on interactions between LSRs and transition metal complexes, both NMR shifts and relaxation rates being studied. Presumably the mode of interaction is by means of bridges formed by the donor atoms of one complex which bond in a labile manner to the metal ion of the second complex. Interactions examined include those between [Eu(fod-d[Pg.1104]

Attempts to interpret the mechanism of ethylene hydrogenation over nickel [96—99] and over platinum catalysts [100,101] in terms of a statistical mechanical approach have not met with any substantial success, partly due to the limitations of the model which must be assumed in order to perform the calculations and partly due to the complexity of the calculations themselves. 

The stability of the olefin complexes seems to be determined by the steric and electronic characters of both the phosphorus ligand and the olefin (22). For example, ethylene complexes have only been isolated for the cases with sterically large ligands such as P(0-o-tolyl)3 and PPh3 however, maleic anhydride forms a stable isolable complex with the smaller P(0-p-tolyl)3 ligand. The nickel-ethylene bond strength is estimated to be 39 kcal/mol based on values of 36 kcal/mol for 1-hexene and 42 kcal/mol for acrylonitrile [when L = P(0-o-tolyl)3] (22). 

Two reports have appeared on the catalyzed reaction of C02 with epoxides to form alkylene carbonates. One of the processes uses phosphine complexes of zerovalent nickel as the catalyst (157), and appears closely related to the more recent isolation of (PCy3)2Ni(C02) (115). Ethylene oxide reacts in benzene under 500 psi pressure of C02 in a stainless steel autoclave at 100°C to form ethylene carbonate with 95% selectivity, (77), using as the catalysts NiL2, L = PCy3 or PPh3. 

Systems (1) enter into class 3 (a PDE point is a PCB). Systems with linear reaction mechanisms belong to both class (2) and class (3) but these classes do not overlap since there are systems without intermediate interactions that do not satisfy the principle of complex balance (e.g. the Eley-Rideal mechanism for CO oxidation on platinum metal). On the other hand, there exist reaction mechanisms containing steps of "intermediate interactions but at the same time always having a PCB (e.g. the Twigg mechanism for ethylene hydrogenation on nickel). 

DFT/MM calculations on ethylene polymerization by nickel diimine complexes have been applied within Car-Parrinello molecular dynamics simulations [40, 41]. A first set of calculations was used to refine the computed energy barrier for the termination step. The enthalpy barrier computed in the calculations described above was 18.6 kcal/mol, a value which decreased to 14.8 kcal/mol at 25 °C in the molecular dynamics calculation, in better agreement with experiment [40]. A second study analyzed the capture of the olefin by the catalyst [41], and found that this process, which has no en-thalpic barrier, has an entropic barrier. 

Complexes of nickel constitute a distinct group of homogeneous alkylalumi-nium-free catalysts for olefin polymerisation. An efficient catalyst for ethylene polymerisation is formed in the reaction of bis(cycloocta-l,5-diene)nickel(0) [Ni(Cod)2] with phosphorus-ylid and triphenylphosphine in toluene solvent [181] ... 

Reactions of 250 with four electrophiles are recorded in Scheme 31. In general, the products are more stable than those from complexes with monodentate ligands. Reaction of 249 with CS2, 249 or 250 with alkynes, and 249-251 with ethylene gives products in which the C6H8 has been lost but its fate has not been determined attempts to trap free cyclohexyne failed.93 Loss of the organic ligand appears to occur more readily in nickel complexes than in those with platinum. 

The synthesis of pure bis(phenylsodium)nickel-ethylene (13) is achieved by reaction of a mixture of NaC6H5/LiC6H5 (Na/Li = 2-4 1) with CDTNi(O) (1) in the molar ratio (NaC6H5 + LiC Hs)/Ni = 2 1 in the presence of ethylene (79). The phenyllithium-containing nickel-ethylene complex 9 remains dissolved while the bis(phenylsodium)nickel-ethylene (13) precipitates out as an orange-red powder. 

Consequently, new dilithium-nickel-olefin complexes with tetra- or pentacoordinated nickel atoms are formed, e.g., the Li2Ni complexes Li2Ni[(CH3)2NCH2CHCHCH2N(CH3)2]3 (18), (LiTMEDA)2Ni(C2H4)3 (19), (LiTMEDA NifCVHw (20), and (LiTHF)2Ni(C4H6)3 (21), by reaction with N,N,N, TV -tetramethy lbutene-2-diamine, ethylene, norbor-nene, or butadiene (14, 31-33). 

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