Nickel complexes tetradentate ligands

The importance of metal coordination compounds in biological systems has led to the study of polydentate Schilf base complexes of cobalt(II), nickel(II), and copper(II) (204, 205). Dimers have been observed in the spectra of complexes of both tri- and tetradentate ligands [e.g., salicylaldehydeand A,A-bis(salicylidene)ethylenediamine]. The parent ions form the base peaks, and the spectra are characterized... 

The tetradentate ligand forms monomeric square planar complexes. Synthetic and kinetic studies reveal that the coordinated mercapto group may be converted into the coordinated thioether function without breaking the metal-sulfur bond. The nucleophilic power of the coordinated mercapto group exceeds that of RSH, but depends on the metal atom. Bridging protects the sulfur atom from alkylation. In the case of nickel(ll), alkylation is accompanied by expansion of the coordination number of the nickel from 4 to 6. Ligand reactions have led to the synthesis of planar ligands completely cydized about the metal ion. 

The indirect electrochemical reduction of alkyl halides is also possible by use of nickel(I) complexes which may be obtained by cathodic reduction of square planar Ni(n)-complexes of macrocyclic tetradentate ligands (Table 7, No. 10, 11) 2 4-248) Comparable to the Co(I)- and Ni(O)-complexes, the Ni(I)-species reacts with the alkyl halide unter oxidative addition to form an organo nickel(III) compound. The stability of the new nickel-carbon bond dominates the overall behavior of the system. If the stability is low, the alkyl group is lost in form of the radical and the original Ni(II)-complex is regenerated. A large number of regenerative cycles is the result. 

The general method of preparation of neutral complexes of the types Ni(N—N)2 and Ni(N4) (N—N and N4 stand for monoanionic bidentate and dianionic tetradentate ligands, respectively, having nitrogen donors) is the direct reaction of a nickel(II) salt and the appropriate... 

Azomethines bear a formal resemblance to azo compounds and many parallels exist in the coordination chemistry of the two series of compounds. Thus the bidentate azomethines (191) behave in a strictly comparable manner to the bidentate azo compounds (20) (Section 58.2.2.1). The isomeric, bidentate azomethines (192), however, form metal complexes which undergo very facile hydrolysis as a result of polarization of the azomethine linkage. The difference between the two types of complex is dramatically illustrated by the results of a study135 of metal complex formation by the bis(azomethine) (193). This cannot function as a tetradentate ligand for steric reasons and reacts with copper, nickel and cobalt halides in cold ethanolic solution to form the five-coordinate complexes (194). Crystallization of these products from ethanol gives the five-... 

Kinetics and Mechanisms of Ligand Replacement Reactions in Trigonal Bipyramidal Nickel(II) Complexes of a Tetradentate Ligand Containing an Apical Phosphorus Donor Atom... 

These cyclophane ligands - featuring two carbene and two pyridine donor groups - do act as tetradentate ligands towards transition metals. Baker et al. [67] reported the respective nickel(II) complex after reaction of the free ligand (bis-imidazolium salt) with nickel(II) bromide in the presence of sodium acetate as base. 

Some iron and nickel cyanide and carbonyl complexes have been reported as models of the [FeNi]-hydrogenase enzymes. The preparation and structures of the trigonal bipyramidal nickel and iron complexes with the tetradentate ligands tris(2-phenylthiol)phosphine (PS3) and tris(3-phenyl-2-thiophenyl)phosphine (PS3 ) have been reported [70, 71]. The nickel carbonyl complex [Ni(PS3 )(CO)] exhibits vco at 2029 cm compared with the value of 1940 em" for the iron earbonyl complex [Fe(PS3 )(CO)]. Both of these complexes lose CO upon oxidation. The use of cyanide in place of carbon monoxide allows for the preparation of both [Fe (PS3)(CN)] and [Fe (PS3 )(CN)] eomplexes. The IR properties of... 

In the presence of nickel(II) a second, tridentate ligand was formed in addition to the tetradentate ligand shown above. The complexes of this ligand, tribenzo[b,f,j][1.5.9]triazacycloduodecine, are octahedral, and the three donor nitrogen atoms occupy one face. The remaining three positions are occupied by two unidentate anions and one water molecule or, in the case of bulky or weakly coordinating anions, by three water molecules. 

The tetradentate ligand, 1,4,8,11-tetraazacyclotetradecane (cyclam), forms solid diamagnetic orange to brown Ni(II) complexes with iodide and perchlorate anions (9). When the anions are changed to chloride and bromide, the solid complexes are paramagnetic (9). However, all the complexes are soluble in water, methanol, and benzene, giving yellow solutions. The yellow color remains on treatment with 12N HCl and excess chloride ion (9). It is interesting to note that this amine is a derivative of a reduction product of the template syntheses between bis(ethylenediamine)-nickel(II) and acetone reported earlier. 

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