5- Coordinate nickel

Coordinate Nickel(ii) Complexes. A considerable number of both trigonal-bipyramidal and square-pyramidal complexes occur and high-(5 = 1) and low-spin (S = 0) examples of each geometry are known.6 Many of the... 

Phase rule studies,vapor pressure measurements,and mass spectrometry are useful in so far as they give indications of stoichiometry but we must not assume that the formation of the compound CsgNiCls, for example, is evidence for 5-coordinate nickel. 

Methyl-coenzyme M reductase participates in the conversion of CO2 to CH4 and contains 6-coordinate nickel(II) in a highly hydrogenated and highly flexible porphyrin system. This flexibility is believed to allow sufficient distortion of the octahedral ligand field to produce low-spin Ni" (Fig. 27.7) which facilitates the formation of a Ni -CHs intermediate. 

First, consider an octahedral nickel(ii) complex. The strong-field ground configuration is 2g g- The repulsive interaction between the filled 2g subshell and the six octahedrally disposed bonds is cubically isotropic. That is to say, interactions between the t2g electrons and the bonding electrons are the same with respect to x, y and z directions. The same is true of the interactions between the six ligands and the exactly half-full gg subset. So, while the d electrons in octahedrally coordinated nickel(ii) complexes will repel all bonding electrons, no differentiation between bonds is to be expected. Octahedral d coordination, per se, is stable in this regard. 

Picolylphenylketone S-benzyldithiocarbazate, 48, yielded paramagnetic [ Ni(48-H)A 2] (A = Cl, Br) and diamagnetic [Ni(48-H)I] [207]. All three compounds are non-electrolytes and the iodo complex is planar while the other two complexes involve sulfur bridging atoms and five-coordinate nickel(II) centers. All three complexes can be converted to monomeric, octahedral complexes by addition of pyridine, 2-picoline or quinoline. 

Nickel(O) reacts with the olefin to form a nickel(0)-olefin complex, which can also coordinate the alkyl aluminum compound via a multicenter bond between the nickel, the aluminum and the a carbon atom of the trialkylaluminum. In a concerted reaction the aluminum and the hydride are transferred to the olefin. In this mechanistic hypothesis the nickel thus mostly serves as a template to bring the olefin and the aluminum compound into close proximity. No free Al-H or Ni-H species is ever formed in the course of the reaction. The adduct of an amine-stabihzed dimethylaluminum hydride and (cyclododecatriene)nickel, whose structure was determined by X-ray crystallography, was considered to serve as a model for this type of mechanism since it shows the hydride bridging the aluminum and alkene-coordinated nickel center [31]. 

Reduction of Ni11 chloride complexes [NiCl2(L)] (L = various diphosphinomethanes, -ethanes, and -propanes) with, for example, potassium naphthalenide in THF gives the corresponding Ni1 chlorides [NiCl(L)].2368 By treatment of (963) with LiNHAr, a terminal amido complex of Ni1 (964) was prepared (Scheme 13).2369 It contains planar three-coordinate nickel and a planar amido ligand with d(Ni—N) = 1.881(2) A. The P,Ni,P and C,N,H planes are orthogonal with a 91° dihedral angle. 

Shoner, S. C., Olmstead, M. M. and Kovacs, J. A. (1994) Synthesis and structure of a water-soluble five-coordinate nickel alkanethiolate complex, Inorg. Chem., 33, 7-8. 

Water NMRD profiles acquired for other complexes and proteins always exhibit the same features of hexaaqua nickel(II). As an example, we report here the profile of the hexa-coordinate nickel(II)-substituted bovine carbonic anhydrase II 54,55) (Fig. 15). As in the aqua complex, (i) the low-field profile is flat, (ii) no dispersion appears, the cOg dispersion being quenched in S = 1 complexes with large static ZFS 56) (see Section I.A.5) and the... 

Most four-coordinate nickel(ll) complexes are square planar. They are of red, brown and yellow color and practically aU are diamagnetic. Some examples are red bis(dimethylglyoximato)nickel(II) and the yellow tetracyanonick-elate(ll) ion, [Ni(CN)4]2-... 

Formation of five-coordinate nickel intermediates via oxidative addition of alkyl halides has been reported (33). This type of... 

Table 8. The electronic spectra of some low-spin five-coordinate nickel(II) complexes )...

Many TM ions can support multiple spin states. Usually, a change in spin is accompanied by significant geometrical changes. For example, high-spin d8 six-coordinate Nickel(II) amine... 

Electronic Absorption and Epr Spectral Data for Four-Coordinate Nickel(I)... 

Electronic Absorption Spectral Data for Five-Coordinate Nickel(I) Complexes with Macrocyclic Ligands... 

Differences in Two Ni-N Bond Distances (A) in Four-Coordinate Nickel(I) Complexes of Various Macrocyclic Ligands0... 

A few three-coordinate nickel(I) complexes are also known. 

The [Ni(PMe3)4]BPlu complex has been unexpectedly obtained through a reductive elimination reaction, by the simple recrystallization in THF of the five-coordinate nickel(II) complex [Ni(Me)(PMe3)4JBPh4.3 ... 

Table 27 Selected References for NMR Spectra of Six-coordinate Nickel(II) Complexes...

Table 29 Magnetic and Spectral Parameters for some Six-coordinate Nickel(II) Complexes3...

In Table 29 spectroscopic properties of selected six-coordinate nickel(II) complexes are shown. 

Many ligand field calculations have been performed on five-coordinate nickel(II) complexes to assign the electronic transitions,518,573"577 but no systematic application of these calculations has been reported. The suggested assignment of the electronic transitions is shown in Tables 31-33. 

Table 32 Magnetic Moments and Spectra) Parameters for some High-spin Five-coordinate Nickel(Il) Complexes with Nearly Square Pyramidal Geometry 1... 

Table 34 Spectral Parameters for some Low-spin Five-coordinate Nickel(II) Complexes with Nearly Trigonal...

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