Coordination complexes, Ni

A template synthesis employing Ni(OAc)2, 2,5-dihydroxy-2,5-dimethyl-1,4-dithiane, and 3,3 -iminobis(propylamine) gave the water-soluble five-coordinate complex [Ni(495)], the crystal structure of which shows trigonal bipyramidal coordination of Ni11 with the central amine and terminal thiolates in plane and the two imino nitrogens in axial positions. Solvatochromism of the complex is interpreted in terms of S" H bonding, which may be of relevance to the catalytic cycle in hydrogenases.1341... 

The fact that the five-coordinate complex [Ni(CN)s] can be detected does indeed explain why substitution reactions of the four-coordinate complex [Ni(CN)4l are fast. The reason is that, for a detectable amount of [Ni(CN)5] to build up in solution, the forward rate constant kf must be numerically dose to or greater than the reverse rate constant... 

Fig. 14e) [116], However, under acidic conditions, the ligand is protonated and the complex rapidly decomposes. Taking a cue from the coordination of hydrogenases, a P2S2-coordinated complex [Ni(bdt)(dppf)] that catalyzes hydrogen evolution at a rate in excess of 10 s with only 265 mV of overpotential [117] and stability on the timescale of hours was reported (Fig. 14f). 

Phosphorus tri-isothiocyanate, P(NCS)3, mp = 4°C, can be made by reaction (9.372). This material will polymerise to a black solid [P(NCS)3l4 and form a yellow coordination complex Ni[P(NCS)3l4,... 

There have been many powder neutron diffraction structural studies of coordination compounds and only a few will be mentioned here. Because of the presence of an unrefmable preferred orientation, it was not possible to solve the structure of the polymeric coordination complex, Ni(l,3-thiazole)2Br2, by powder X-ray diffraction." In contrast, a Rietveld refinement of powder neutron diffraction data did yield a pseudooctahedral structure doubly linked by bridging bromide ions into infinite linear chains. 

When bulky Bu or adamantyl groups are placed in the 2,5 positions of NHC ligands, stable three-coordinate complexes Ni(CO)2(NHC) are obtained. These are rare examples of three-coordinate nickel carbonyl complexes 6. 

A related three-coordinate dicarbonyl-carbene complex can be obtained by reaction of Ni(CO)4 with the carbodi-phosphorane double ylide Ph3=C=PPh3. The reaction of Ni(GO)4 with Ph3=C=PPh3 in toluene gives the tricarbonyl complex Ni G(PPh3)2 (CO)3, but in THF solvent the deep red 16-electron three-coordinate complex Ni G(PPh3)2 (GO)2 was obtained. Both the three- and four-coordinate compounds Ni G(PPh3)2 (GO) , n = Z, 3) were structurally characterized by X-ray diffraction. ... 

Because the electron-counting paradigm incorporates the 18-electron rule when appHed to transition-metal complexes, exceptions can be expected as found for classical coordination complexes. Relatively minor exceptions are found in (Tj -C H )2Fe2C2BgHg [54854-86-3] (52) and [Ni(B2QH22)2] A [11141-32-5] (53). The former Q,n electrons) is noticeably distorted from an idealized stmcture, and the latter is reminiscent of the and complexes discussed above. An extremely deficient electron count is obtained for complexes such as P7036-06-9] which have essentially undistorted... 

Planar-octahedral equilibria. Dissolution of planar Ni compounds in coordinating solvents such as water or pyridine frequently leads to the formation of octahedral complexes by the coordination of 2 solvent molecules. This can, on occasions, lead to solutions in which the Ni has an intermediate value of jie indicating the presence of comparable amounts of planar and octahedral molecules varying with temperature and concentration more commonly the conversion is complete and octahedral solvates can be crystallized out. Well-known examples of this behaviour are provided by the complexes [Ni(L-L)2X2] (L-L = substituted ethylenediamine, X = variety of anions) generally known by the name of their discoverer I. Lifschitz. Some of these Lifschitz salts are yellow, diamagnetic and planar, [Ni(L-L)2]X2, others are blue, paramagnetic, and octahedral, [Ni(L-L)2X2] or... 

The 1,2-azaphosphole i7 (P)-coordinated complexes 83 and 84 follow from the corresponding ligand and [(i -allyl)NiCl]2 and [(1,5,9-cyclododecatriene)Ni], respectively (98ICA(270)273). 

To remove an ion, we can use the fact that many metal cations are Lewis acids (Section 10.2). When a Lewis acid and a Lewis base react, they form a coordinate covalent bond and the product is called a coordination complex. In this section, we consider complexes in which the Lewis acid is a metal cation, such as Ag+. An example is the formation of Ag(NI 1,)2+ when an aqueous solution of the Lewis base ammonia is added to a solution of silver ions ... 

There is one striking group of exceptions to the otherwise almost unbroken success of Kepert s approach. No model predicated solely upon the repulsions between monodentate ligands (or between bonds) can account for the planarity of some four-coordinate complexes. Yet hundreds of planar (f complexes like [Ni(CN)4] or [PtCl4] are known. Clearly, Kepert s model is to be augmented and we discuss this matter further in Chapter 7. 

An X-ray crystal-structure determination (370) revealed that mixed, nickel coordination exists in the trimeric complex, [Ni(S2CPh)2]3. The trimeric structure is similar to that found (371) in [Pd(PhCS2>]. The structure contains one molecule of type A linked centrosymmetrically through short Ni-S bridges to two molecules of type B, the three molecules being closely parallel (XXVII). Bonamico and co-workers (372)... 

Complex ions, also called coordination complexes, have well-defined stoichiometries and structural arrangements. Usually, the formula of a coordination complex is enclosed in brackets to show that the metal and all its ligands form a single structural entity. When an ionic coordination complex is isolated from aqueous solution, the product is composed of the complex ion and enough counter-ions to give a neutral salt. In the chemical formula, the counter-ions are shown outside the brackets. Examples include the sulfate salt of [Ni (NH3)g, ... 

Cyanide binds to a series of Ni complexes with N302 chelate ligands, and EPR spectra of the adducts indicate quasi-axial symmetry with a g pattern typical of low-spin, six-coordinate complexes with axial elongation and with a 2Ay ground state. No direct spin interaction of the unpaired electron with the carbon atom of the cyanide takes place.70... 

---