Ni-C bonds

Carbon monoxide insertion into Ni—C bonds has been postulated in carbonylation reactions involving (7r-C3H5NiX)2 (X = C1, Br, or I) (112, 123a). 

In the general case, an incoming nucleophile would be expected to be favoured by (i) a high basicity consistent with ( ) a high polarizability, and the metal complex to favour its approach if (in) it contains electron-acceptive, or B class ligands. An interpretation of the available data may be essayed on these lines. The infrared data upon Ni(CO)4 are consistent with a weakening of the C-O bond , and it would be of interest to examine the solvent effect upon the Ni-C bond. 

The thiolato complex 97 that was postulated as the active catalytic species in the reaction was prepared from 96 and the thiol in the presence of NEtj. Certain analogues of 97 (NHC = Mes, SIMes, IPr, SIPr R = Ph) have also been independently synthesised, isolated and fully characterised. A plausible mechanism for the hydrothiolation involves insertion of the alkyne into the Ni-SR bond forming the (non-isolable) p-thioalkenyl complex, from which the product can be released via alkanolysis of the Ni-C bond by the thiol and regeneration of the active catalyst 97 [84]. 

A series of complexes of type [Ni(T] -allyl)Cl(NHC)] highlight the important influence of the NHC on the reactivity of the resulting complex towards O. It was shown that 0,-activation is disfavoured when the rotation around the Ni-C, bond is restricted. On the other hand, with complexes displaying free rotation around the Ni-Cj, bond, the complexes react cleanly with O. The overall reaction results in the oxidation of the allyl group and the formation of hydroxy-bridged dimers (Schane 10.4) [14,15]. 

That the sequence shown in Scheme 3 is not the only pathway available for H—NiY formation is indicated by the isolation of 1,3-cyclooctadiene from the reaction products of the dimerization of propene with the n-cyclooctenylnickel system (25) (80) it seems reasonable that the H—NiY species 22 in this case is at least in part formed through direct elimination from 25 without prior monomer insertion into the Ni—C—bond [Eq. (6)] ... 

Similar to 67, the oxanickelacycle 69 prepared from 3-hexyne, Ni(cod)2/bipy, and C02 also acted as a nucleophilic reagent. The reaction of 69 with p-tolyl disulfide took place at the Ni-C bond to give vinyl sulfide 70 in 53% yield after esterification (Scheme 23).37 In the case of 2-bromopropiophenone as an electrophile, the cycloadduct 71 was obtained in 47% yield after acidic workup. 

Insertion of ethylene into the Ni-C bond in 3a leads to the alkyl complex 4a via the transition state TS[3a-4a] with a barrier [13a] of 17.5 kcal/mol relative to 3a It is worth to note that in TS[3a-4a both ethylene and the a-carbon of the growing (propyl) chain are situated in the N-Ni-N plane. For the corresponding palladium complex the insertion barrier [13c] is somewhat higher at 19.9 kcal/mol. 

Only in homoleptic M(L)2 (L = 1,3-dimesitylimidazolin-2-ylidene) of zero-valent nickel and platinum significantly shorter metal-carbon bonds for NHCs and, thus, metal-to-ligand back donation can be observed. The Ni-C bond length is about 0.15 A shorter than in [Ni(CO)2(L)2] (L = 1,3-dimesitylimidazolin-2-ylidene) which cannot be explained exclusively by the change of the coordination number. 

Strengthening of the Ni-C bond by electron charge donation of a trans phosphine ligand in the bisphosphine complex (equation 18) retards the elimination of CO prior to the oxidative step (30, 31). This is not the case for the phosphine nickel tricarbonyl (equation 19) where carbon monoxide is easier to eliminate (32). 

At room temperature, ethylene is adsorbed, without fragmentation, by opening of the double bond and formation of two Ni—C bonds similarly, benzene appears to be adsorbed by six-site (Ni—C) attachment. Adsorption of saturated hydrocarbons involves fragmentation with the formation of Ni—H, as well as Ni—C, bonds. At 100° ethylene also undergoes fragmentation. 

The second-molecular system that has recently been studied is CO in a c(2 X 2) arrangement on the Ni(lOO) crystal face . It appears from LEED that this molecule is bound by its carbon end to one nickel atom with a Ni-C bond length of 1.8 0.1 A, cf. Fig. 7.4. The carbon-end bonding configuration has long been expected from UPS and IR evidence and HREELS confirms bonding to a single nickel atom. However, the CO internuclear axis is observed not to be perpendicular to the surface but tilted by 34 10° from the surface normal. But photoemission results do favor a perpendicular position of the molecule. 

Organonickel(II) species are believed to be formed during the reaction between [Ni(TMC)] and primary alkyl halides, and subsequently undergo hydrolysis with cleavage of the Ni—C bond. Kinetic data measured in the presence of excess alkyl halide indicate a rate law -dlNi1 (TMC)+]/cft = MNi (TMCr][RX]. The rate constants increase for R and X in the order methyl < primary < secondary < allyl < benzyl halides and Cl < Br < I (133, 140). This suggests that the rate-determining step is electron transfer from the Ni(I) complex to R—X via an inner-sphere atom-transfer mechanism (143). 

With this structure the nickel atom lias achieved the krypton electron configuration its outer shell contains five unshared pairs (in the five M orbitals) and five shared pairs (occupying the 4s4p3 tetrahedral bond orbitals). The Ni—C bond length expected for this structure is about 2.16 A, as found by use of the tetrahedral radius 1.39 A obtained by extrapolation from the adjacent values in Table 7-13 (Cu, 1.35 A Zn, 1.31 A). 

With the assumption that the 4100 structures have equal weights, the carbon-carbon bonds in the ring are calculated to have bond number n = 1.173 and the nickel-carbon bonds to have n = 0.439, with 34.6 percent d character for the nickel bond orbitals. The number of unshared pairs on the nickel atom is 2.89. The formal charge on the nickel atom is —0.64 of this, the 4.39 Ni—C bonds, with 12 percent ionic character, would provide the opposite charge -f 0.53, leaving the nickel atom essentially neutral (charge —0.11). 

A large number of nickel(O) phosphine complexes with rj2-bonded unsaturated organic molecules have been reported. Here we will review relevant examples of complexes with f 2-bonded molecules which contain a number of Ni—C bonds not exceeding the number of bonds from nickel to non-carbon atoms (usually phosphorus). The early examples (up to 1972) of complexes with alkenes have been extensively reviewed.11... 

Complex Colour v(CN) stretch (Nujol mull) (cm-1) Ni—C bond distance (pm) Ref. 

The reaction of CO with some of the preceding organometallic compounds is rapid at room temperature and pressure and insertion of CO into the Ni—C bond results (equation 175).1445 In the case of the np3 ligand the first product isolated is a solid solution of the acyl derivative of nickel(II), [Ni(COR)(np3)]+, and a carbonyl complex of nickel(I), [Ni(CO)(np3)]+, in a 1 1 ratio. When this solid solution is dissolved in THF and EtOH, the pure acyl derivative (190) resulted. The acetyl derivative spontaneously loses CO on exposure to air restoring the original methyl derivative. 

Addition of excess CH3I to a solution of [Ni (tmc)]+ results in the rapid loss of the absorption (A = 360 nm, e = 4 x 103 M-1 cm-1) and appearance of a less intense band at A = 346 nm. A subsequent slower reaction gives rise to the weaker absorbance profile of [Ni"(tmc)]2+. The data are interpreted in terms of the formation of an organo-nickel(II) species followed by a slower hydrolysis with breaking of the Ni-C bond. Kinetic studies under conditions of excess alkyl halide show a dependence according to the equation — d[Ni1(tmc)+]/cft = 2 [Ni(I)][RX]. The data have been interpreted in terms of a ratedetermining one-electron transfer from the nickel(I) species to RX, either by outer-sphere electron transfer or by halogen atom transfer, to yield the alkyl radical R. This reactive intermediate reacts rapidly with a second nickel(I) species ... 

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