Complex square-planar nickel hydride

A square-planar nickel hydride complex is suggested as the catalytic species [589]. In the first step, the nickel hydride catalyst adds across the double bond of propylene to give two intermediates, namely, a propyl nickel and isopropyl nickel complex. Both of these intermediates can react further with propylene by insertion of the double bond into the nickel-carbon bond, resulting in formation of four more intermediates. ( -Elimination of nickel hydride from these intermediates produces the possible products of propylene dimerization, namely, 4-methyl-1-pen-tene, cis- and trans-4-methyl-2-pentene, 2,3-dimethyl-l-butene, n-hexene, 2-hexene, and 2-methyl-l-pentene. Terminal unbranched olefins are rapidly isomerized under the influence of catalyst by a process of repeated nickel hydride addition and elimination to the internal olefins. Therefore, under ordinary reaction conditions the yield of 4-methyl-l-pentene is low. 

The formation of cationic nickel hydride complexes by the oxidative addition of Brdnsted acids (HY) to zero-valent nickel phosphine or phosphite complexes (method C,) has already been discussed in Section II. Interesting in this connection is a recent H NMR study of the reaction of bis[tri(o-tolyl)phosphite]nickelethylene and trifluoroacetic acid which leads to the formation of a square-planar bis[tri(o-tolyl)phosphite] hydridonickel trifluoroacetate (30) (see below) having a cis arrangement of the phosphite ligands (82). 

The hydrido complexes are diamagnetic, and are square planar or five-coordinate. Their stability, in general, increases with the number of coordinated phosphines. In the complex [Ni(BH4)(H)(PCy3)2] (153) the nickel atom is coordinated in the equatorial positions by two hydrogens of the borohydride and by one hydride anion. 

Chlorohydridobis(tricyclohexylphosphine)nickel is a yellow-brown solid. It is thermally stable at ambient temperature but reacts with air. It is very soluble in benzene, tetrahydrofuran, and dichloromethane and is soluble in diethyl ether and petroleum ether. Carbon tetrachloride, carbon disulfide, and chloroform decompose the complex. The infrared spectrum shows a sharp v(Ni—H) band at 1916 cm-1 (KBr disk and Nujol mull). The high-field H nmr spectrum in benzene solution has a triplet (1 2 1) at t34.6 (TMS) with JpH 73.5 Hz. The splitting is caused by the coupling of the hydride proton with two equivalent 31P nuclei. This is consistent with a trans square-planar configuration. 

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