Nickel tetrahedral complexes

Tetrakisligand nickel(0) complexes have tetrahedral stmctures. Electronic stmctures have been studied and conformational analysis performed. Quantitative equiUbria measurements of the ligands in these complexes imply a dominant role for ligand steric effects when the complexes are employed as catalysts (94). 

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,... 

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. 

The change from high- to low-spin configurations is necessarily discontinuous. A given complex is either on one side of the divide or the other. We conclude this section with a look at how the steric role of the d shell can affect angular geometries within a series of just high-spin, nominally tetrahedral nickel(ii) complexes. 

A quantitative consideration on the origin of the EFG should be based on reliable results from molecular orbital or DPT calculations, as pointed out in detail in Chap. 5. For a qualitative discussion, however, it will suffice to use the easy-to-handle one-electron approximation of the crystal field model. In this framework, it is easy to realize that in nickel(II) complexes of Oh and symmetry and in tetragonally distorted octahedral nickel(II) complexes, no valence electron contribution to the EFG should be expected (cf. Fig. 7.7 and Table 4.2). A temperature-dependent valence electron contribution is to be expected in distorted tetrahedral nickel(n) complexes for tetragonal distortion, e.g., Fzz = (4/7)e(r )3 for com-... 

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. 

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... 

Kinetic evidence obtained for intramolecular proton transfer between nickel and coordinated thiolate, in a tetrahedral complex containing the bulky triphos ligand (Pl PCE CE PPh to prevent interference from binuclear p-thiolate species, is important with respect to the mechanisms of action of a number of metalloenzymes, of nickel (cf. urease, Section VII. B.4) and of other metals (289). 

There have been few studies of substitution in complexes of nickel(II) of stereochemistries other than octahedral. Substitution in 5-coordinated and tetrahedral complexes is discussed in Secs. 4.9 and 4.8 respectively. The enhanced lability of the nickel(II) compared with the cobalt(II) tetrahedral complex is expected from consideration of crystal field activation energies. The reverse holds with octahedral complexes (Sec. 4.8). 

It is in this oxidation state that the most striking advances have been made in the stereochemistry of these elements. The search for tetrahedral nickel(II) complexes, eventually successful in its original aim, has allowed certain other discoveries to be made which have opened up new ideas in stereochemistry. The subject is very closely connected with ligand-field theory, and the essentials of this theory are summarized here as they apply to d8-systems. Tor the reader who is unfamiliar with ligand-field theory, more complete accounts can be found in several review articles (10, 22, 64, 109, 137, 180, 193, 196, 197). 

Ligand-field theory predicted (10, 22) that tetrahedral nickel(II) complexes should be unstable with respect to octahedral ones, at least so long as the two extra ligands were available. This arises because, if one accepts the d-orbital center of gravity as an energy-zero (a point which should be raised more often), the crystal-field stabilization of an octahedral complex works out to be 0.84 A greater than that of a tetrahedral complex with... 

Liehr has shown (142) that the orbital degeneracy of the ground state in a tetrahedral nickel(II) complex may be lifted by spin-orbit coupling. This means that these complexes may not be liable to Jahn-Teller distortion as has been thought for some time. Such coupling would also have the effect of splitting all transitions into several components, the exact number... 

Platinum(II) compounds are to be found only towards the bottom of this scheme, palladium(II) reaches further up, its fluoride belonging to the tetragonal, paramagnetic class. Nickel(II) complexes cover the whole range of behavior, and may in addition be tetrahedral. 

Nickel(II) in tetrahedral symmetry has an orbitally degenerate ground state and the magnetic moments of tetrahedral complexes are expected to be substantially higher than those of six-coordinate complexes because of the larger orbital contribution. The magnetic moments are usually found to be in the range 3.3-4.0 BM at room temperature and tend to zero at very low temperatures. 

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