Nickel complexes square-planar structures

Following the aforementioned pioneering work, many other organometallic complexes of nickel(II) were prepared using different synthetic procedures. Complexes containing one molecule of tertiary phosphine were prepared using methods similar to that reported in equations (142) and (143) and Scheme 13.1193-11951219-1223 All of these diamagnetic compounds have the square planar structure exemplified by (155).1193... 

Transition metal hydroxyoxime complexes have been reviewed very recently.2507 Their use in both analytical chemistry and extraction metallurgy is well known. The square planar structure of the bis chelate complex NiL (347) with the deprotonated 2-hydroxybenzaldoxime (HL) is typical of this series of nickel complexes.2508 Their bis adducts, NiLJ, with bases such as py, substituted pyridines and cyclomethyleneimines, are six-coordinate.2509 The acyl oxime (H2L) complexes are similar to the aforementioned complexes being either square planar bis chelates Ni(HL)2 (348) or octahedral bis adducts, Ni(HL)2B2.2507 When the acyl oxime acts as a dibasic ligand L, the corresponding (NiL) complexes are insoluble and involve extensive polymerization. 

Bis(salicylaldoximato)nickel(II) is diamagnetic in the solid state, but becomes partially paramagnetic in solution in chloroform157 and aqueous dioxane.151 This is likely to be due to molecular association or solvation,158 rather than to the adoption of a distorted tetrahedral structure in solution.151 The nickel(II) complex readily forms pseudo-octahedral bis-adducts in the presence of large amounts of amines,159 160 although analogous complexes of commercial orffto-hydroxy-oximes have been reported to retain the square-planar structure in hydrocarbon solution in the presence of moderate amounts of ammonia or aliphatic amines.161... 

Nickel(I) complexes of N4 macrocycles can be prepared by reduction of the corresponding Ni11 complexes with sodium amalgam. They possess more or less distorted square-planar structures.19 By contrast, the one-electron reduction of Ni porphyrin complexes may result in Ni1 porphyrins or Nin jr-anion radicals, depending on the reaction conditions.20 Complexes of this kind are useful models for the Ni sites in certain metalloenzymes (see below). 

Experimental evidence supporting the square planar structure of first row transition metal ions was presented in 1931 [Ni(CN)4] complexes were found to be diamagnetic, and K2Ni(CN)4-H20 and K2Pd(CN)4-H20 were found to be isomorphous (75, 74). Pauling s prediction (75) that bivalent nickel complexes could be either square planar dsp hybridization) or tetrahedral sp hybridization) stimulated interest in the square con-... 

The second series of related materials was based on the (3,4-didodecyloxyphenyl)diaminodi-oxime derivative (73). Vicinal dioximes are capable of coordinating through N,N or N,0 sites of the oxime groups. Upon the complexation, the ligands fold back, and the transition metal complexes are V,V-coordinated with a square-planar structure. Both isomeric forms were selectively isolated for the nickel complexes, but only the anii-isomer for the palladium complex. The two uwti-complexes complexes exhibited a Colh phase ((73) M = Ni Cr 78 Colh H7 I M = Pd Cr 80 Colh 131 I). Most astonishing, however, was the observation of a Colh phase for the isomeric amphi-mckel complex ( , Z-isomer), between 66 °C and 145 °C. Not only was the existence of mesomorphism surprising and unexpected, but the mesophase stability also increased with respect to the anti-complex. 

As was stated earlier, four-coordinated complexes have either a tetrahedral or a square planar structure. The geometry observed in a particular system depends on the metal and also on the ligands. Thus complexes of beryllium(ii) are always tetrahedral, whereas nickel(ll) forms tetrahedral [NiBr4] and square planar [Ni(CN)4] ". For certain ligands the stabilities of the two structures may not differ greatly in such cases both forms may be obtained and these are called conformational isomers. Examples of such isomers for nickel(ii) and cobalt(ii) are... 

The macrocyclic product [Ni(L54)]I is isolated through refluxing an aqueous solution of L52 and nickel(II) acetate with the subsequent addition of an aqueous solution of Nal (Eq. 2,15) [23], The diamagnetism and electronic absorption spectrum of [Ni(L54)]+ indicate that it has a square-planar structure. The peak with the maximal m/z in the mass spectrum of the iodide corresponds to the [Ni(L54)]+ cation. It should be noted that, in the mass spectra of a number of other macro-cyclic coordination compounds of the [ML]X type, the most intense peak in the high-mass number range is [ML]+ as, for example, in case of the nickel( II) complex with the 13-membered macrocycle derived from 3,7-diaza-l,9-diaminononane [24]. 

A crystal-structure determination on [Ni(PhCH2CS2)2] showed evidence of a Ni-Ni bond (Ni—Ni distance, 256 pm) in a bridging, acetate-cage, binuclear complex (363). Each nickel atom is 5-coordinate and is in a tetragonally distorted, square-pyramid spectroscopic evidence for a Ni-Ni bond has been obtained (364). The polarized crystal spectra showed more bands than predicted for a mononuclear, diamagnetic, square-planar nickel(Il), and the spectra are indicative of substantial overlap of the d-orbitals between the two nickel atoms. The bis(dithiobenzation)nickeKII) complex was found to exhibit unusual spectrochemical behavior (365). 

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