Nickel planar complexes

Although less numerous than the square-planar complexes, tetrahedral complexes of nickel(II) al.so occur. The simplest of these are the blue (X = Cl, Br, I) ions,... 

Figure 27.6 Some typical planar complexes of nickel (11) (a) Ni(Mcft-acac)2). (b) Ni(Me-sal)2] and (c) [Ni(dmgll)2. (Note the short 0-0 distance which is due to strong hydrogen bonding.)...

Monomer-oligomer equilibria. [Ni(Me-sal)2], mentioned above as a typical planar complex, is a much studied compound. In pyridine it is converted to the octahedral bispyridine adduct (/zsoo = 3.1 BM), while in chloroform or benzene the value of is intermediate but increases with concentration. This is ascribed to an equilibrium between the diamagnetic monomer and a paramagnetic dimer, which must involve a coordination number of the nickel of at least 5 a similar explanation is acceptable also for the paramagnetism of the solid when heated above 180°C. The trimerization of Ni(acac)2 to attain octahedral coordination has already been referred to but it may also be noted that it is reported to be monomeric and planar in dilute chloroform solutions. 

Square planar complexes, in which the four bonds are directed toward the comers of a square, are more common. Certain complexes of copper(II) and nickel(II) show this geometry it is characteristic of die complexes of Pd2+ and Pt2+, including Pt(NH3)42+. 

In addition to the tetrahedral and octahedral complexes mentioned above, there are two other types commonly found—the square planar and the linear. In the square planar complexes, the central atom has four near neighbors at the corners of a square. The coordination number is 4, the same number as in the tetrahedral complexes. An example of a square planar complex is the complex nickel cyanide anion, Ni(CN)4-2. 

It has square planar coordination (Pd-N 2.010-2.017 A) similar to the value of 2.009 A in the tetraphenylporphyrin analogue, prepared by a similar route. As with nickel, macrocycle complexes can be made by in situ template... 

Splitting of d-orbitals in square planar complexes of copper(II), nickel(II) and cobalt(II). Y. Nishida andS. Kida, Coord. Chem. Rev., 1979, 27, 275-298 (94). 

The M(dioxime-BR2)2 class of complexes 120-122 with four-coordinate metal ions in a square-planar environment has attracted attention in view of possible columnar M M interactions that may result in interesting semiconducting properties in the solid state [182]. Therefore, a series of nickel(II) complexes... 

Other metals can also be used as a catalytic species. For example, Feringa and coworkers <96TET3521> have reported on the epoxidation of unfunctionalized alkenes using dinuclear nickel(II) catalysts (i.e., 16). These slightly distorted square planar complexes show activity in biphasic systems with either sodium hypochlorite or t-butyl hydroperoxide as a terminal oxidant. No enantioselectivity is observed under these conditions, supporting the idea that radical processes are operative. In the case of hypochlorite, Feringa proposed the intermediacy of hypochlorite radical as the active species, which is generated in a catalytic cycle (Scheme 1). 

With 2-formylpyridine S-methyldithiocarbazate, planar, diamagnetic nickel(II) complexes with stoichiometry [Ni(6-H)A], where A = Cl, Br, I, and NCS, have been isolated [126]. Also, paramagnetic, octahedral [Ni(6-H)2] has been prepared and spectrally characterized. These complexes have NNS coordination as does the related S-benzyldithiocarbazate, 24, in [Ni(31-H)A], A = Cl, Br [165]. 

The Ni—P bond is one of the basic motifs in Ni-based catalysis and Ni organometallic chemistry. The earlier work on phosphine-nickel complexes has been reviewed.636 The complex [Ni(dppm)Cl2] with the Ph2PCH2PPh2 (dppm) ligand is square planar. Complexes of formula... 

Dithioacetyacetone (sacsac) forms square planar complexes with Ni11, the chelate rings being six membered. A comparative IR, NMR, and13C NMR study of nickel complexes with sacsac... 

Nickel(II) complexes of (505) exhibit spin equilibria in solution.1355 With the bidentate analogues (506), complexes [Ni(506)2] have been isolated.1356 When Rj = Ph, the complex is tetrahedral in solution. It has a temperature independent magnetic moment of 2.75pB- When R = Me, the complex exhibits square planar-tetrahedral equilibrium in solution. Both are, however, diamagnetic in the solid state. 

Nickel(II) complexes of type (599) unsaturated 14-membered tetrazamacrocycles react with the anion of nitromethane under basic conditions to give the 5-nitromethyl derivatives (600), and the square planar diperchlorates (601) upon treatment with HC104 (Equation (19)) 151... 

Tinnemans et al.132 have examined the photo(electro)chemical and electrochemical reduction of C02 using some tetraazamacrocyclic Co(II) and Ni(II) complexes as catalysts. CO and H2 were the products. Pearce and Pletcher133 have investigated the mechanism of the reduction of C02 in acetonitrile-water mixtures by using square planar complexes of nickel and cobalt with macrocyclic ligands in solution as catalysts. CO was the reduction product with no significant amounts of either formic or oxalic acids... 

Nickel(II) dithiocarbamates, Ni(S2CNRR )2, are monomeric, square planar complexes. A less common dinuclear complex is the thiolato bridged [Ni(S2CNHMe)(p-SMe)]2, in which the dimers are self-assembled into tetra-meric supermolecules through weak axial Ni- S secondary bonds.397... 

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 square-planar complex (34) NiCI2-(P-/i-Bu3)2 was a better catalyst than the tetrahedral complex NiBr2 (PPh3)2 for hydrosilation of styrene with trichlorosilane at temperatures of 150°-170°C. A nickel(0) complex, Ni[P(OPh)3]4, was as good as NiCl2(NC5H5)4, which was best among known nickel catalysts for this reaction. Addition of copper(I) chloride... 

Since the octahedral and tetrahedral configurations have the same number of unpaired electrons (that is, 2 unpaired electrons), we cannot use magnetic properties to determine whether the ammine complex of nickel(II) is Octahedral or tetrahedral. But we can determine if the complex is square planar, since the square planar complex is diamagnetic with zero unpaired electrons. 

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