Nickel-carbon distance

The cluster catalyzes hydrogenation (20°C and 3 atm) of dialkyl- and diarylacetylenes to the c/s-olefins via unsaturate routes, likely involving Ni4(CNR)6(RC=CR) and Ni4(CNR)4(RC==CR)3 (391, 392). The acetylenes in the latter complex bridge three nickel centers, and increase of the acetylenic carbon-carbon bond distance is considered to enhance reduction by hydrogen (392, 393). 

Protonation of Ni(CNXyl)(triphos) to afford the carbene complex results in oxidation of Ni(0) to Ni(ii). This is evident in the change of coordination geometry from tetrahedral to square planar. The G(5)-N(l) bond distance in the carbene complex (1.268(5) A) is shorter than a typical G-N single bond (1.472(5) A), but longer than the G-N triple bond of isocyanides (1.157(5) A). The three substituents on the carbene carbon are coplanar, consistent with the sp character of the carbon atom. The nickel-carbon bond distance of the carbene complex /(Ni(l)-G(5)), 1.860(4) A, is 0.07 A longer than the Ni-G distance of the Ni(0) isocyanide complex. Overall, the metrical parameters fall into a range that reflects essentially no 7r-carbene character and significant iminium formyl character. 

If it is assumed that 2,2 -bipyridine is bonded to the catalyst by both nitrogen atoms, then the position of the chemisorbed molecule on the metal is rigidly fixed. Unless two molecules of this base can be adsorbed at the required distance from each other and in an arrangement which is close to linear, overlap of the uncoupled electrons at the a-position cannot occur. The failure to detect any quaterpyridine would then indicate that nickel atoms of the required orientation are rarely, if ever, available. Clearly the probability of carbon-carbon bond formation is greater between one chemisorbed molecule of 2,2 -bipyridine and one of pyridine, as the latter can correct its orientation relative to the fixed 2,2 -bipyridine by rotation around the nitrogen-nickel bond, at least within certain limits. 

The reaction of [Ni(ethene)3] with a hydride donor such as trialkyl(hydrido)-aluminate results in the formation of the dinuclear anionic complex [ Ni(eth-ene)2[2l 11 [22]. The nickel(O) centers in this complex are in a trigonal planar environment of two ethene molecules and a bridging hydride ion, with the ethene carbons in the plane of coordination. The two planes of coordination within the dinuclear complex are almost perpendicular to each other, and the Ni-H-Ni unit is significantly bent, with an angle of 125° and a Ni-Ni distance of 2.6 A [22],... 

A. This distance is the distance between the hydrogen on the first carbon and the third carbon atom in an aliphatic chain. This critical condition of the catalyst is satisfied by sulfuric acid, phosphoric acid, silicic acid, aluminosilicic acid, perchloric acid, moist aluminum chloride ( HAICI4 ), and partially hydrogenated nickel. 

The [Ni(CN)4]2 anion is one of the most stable nickel(II) complexes and an overall formation constant as high as about 1030 has been determined.627,62 The structure of the complex is square planar with the nickel(II) bound to carbon atoms of cyanides and with linear Ni—C—N linkages (Table 37).629 630 The planar [Ni(CN)4]2 units are stacked in columns in the crystal lattice with Ni—Ni interlayer distances as short as 330 pm. C-bonded CN- is a strong field donor and the electronic spectrum of [Ni(CN)4]2 shows two weak d-d bands at 444 and 328 nm. 

EXAFS studies showed that a nickel atom in a sulfided Ni-Mo catalyst supported on j/-A1203 or on carbon is surrounded by four or five sulfur atoms at a distance of 2.2 A, by one or two molybdenum atoms at a distance of 2.8 A, and by one nickel atom at a distance of 3.2 A (20). These data are consistent with a model in which the nickel atoms are located at the M0S2 edges in the molybdenum plane in a square pyramidal coordination. The nickel atoms are connected to the M0S2 by four sulfur atoms, and depending on the H2S partial pressure a fifth sulfur atom may be present in the apical position in front of the nickel atom (Fig. 4). Recent density functional... 

Figure 4-26. Unconstrained MD simulations for methyl acrylate bound to the palladium/nickel diimine complex through the oxygen (top-right/bottom-right) and the C=C (top-left/bottom-left) functionalities. The three panels in each of the four graphs represent variations in the metal-carbon (top two panels) and metal-oxygen (bottom panel) distances. The simulations were carried out at 300 K for all the systems, and 700 K for the local minima, as indicated...

The non-metallic atoms (C and H) are described by the 6-31G basis set of double zeta quality with p polarization functions in hydrogen atoms and d polarization functions in carbon atoms. In aU the clusters, the nearest-neighbour distances were taken from the bulk and are 2.77483 A for platinum, 2.75114 A for palladium and 2.49184 A for nickel. These clusters form compact sections of the corresponding ideal surfaces. 

---