Nickel complexes polydentate

Nickel forms a large number of complexes with various anions (monoden-tate, bidentate, and polydentate) and many neutral ligands. The most common coordination numbers of the metal in these complexes are six and four while the metal is usually in +2 oxidation state, Ni2+. Also, some complexes of three and five coordinations exist. Several zero valent nickel complexes, such as nickel tetracarbonyl, and a number of substituted carbonyl complexes are well known. 

Nickel(H) complexes with ligands containing mixed donor atoms, in general N, O and S, are innumerable. In die present section we will mention complexes which are the archetypes or are suitable for the description of the properties of all complexes of the same type. Nickel complexes with hybrid polydentate ligands containing either P or As have been discussed in Section 50.5.4. 

Table 3.2. Values of the Partial Stability Constants of Nickel Complexes of Polydentate... 

The Chelate Effect and Polydentate Ligands 147 Table 8-1. Stability constants for some nickel(ii) complexes of ammonia and 1,2-diaminoethane. 

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... 

Eichhom and his co-workers have thoroughly studied the kinetics of the formation and hydrolysis of polydentate Schiff bases in the presence of various cations (9, 10, 25). The reactions are complicated by a factor not found in the absence of metal ions, i.e, the formation of metal chelate complexes stabilizes the Schiff bases thermodynamically but this factor is determined by, and varies with, the central metal ion involved. In the case of bis(2-thiophenyl)-ethylenediamine, both copper (II) and nickel(II) catalyze the hydrolytic decomposition via complex formation. The nickel (I I) is the more effective catalyst from the viewpoint of the actual rate constants. However, it requires an activation energy cf 12.5 kcal., while the corresponding reaction in the copper(II) case requires only 11.3 kcal. The values for the entropies of activation were found to be —30.0 e.u. for the nickel(II) system and — 34.7 e.u. for the copper(II) system. Studies of the rate of formation of the Schiff bases and their metal complexes (25) showed that prior coordination of one of the reactants slowed down the rate of formation of the Schiff base when the other reactant was added. Although copper (more than nickel) favored the production of the Schiff bases from the viewpoint of the thermodynamics of the overall reaction, the formation reactions were slower with copper than with nickel. The rate of hydrolysis of Schiff bases with or/Zw-aminophenols is so fast that the corresponding metal complexes cannot be isolated from solutions containing water (4). 

Five-coordination is now quite common in nickel(II) complexes and many polydentate ligands such as polyamines, salicylaldimines, polyarsines and polyphosphines have been designed with the purpose of favouring this stereochemistry.7,8 However, five-coordinate complexes with monodentate ligands ([Ni(CN)s]3 and [Ni(OAsMe)s]2+) are also known. 

Numerous nickel(II) complexes with a variety of polydentate amines have been described. Selected examples of such complexes are collected in Table 42. In general, solid complexes have been easily obtained by direct synthesis from nickel salts and the appropriate ligand using H20, MeOH, EtOH or butanol as reaction medium. Most of the complexes with the fully TV-alkyl-substituted ligands are conveniently prepared under anhydrous conditions. 

Substrates 1-4, containing Rp substituents (Cf. eqs. 1-3) have been prepared according to literature procedures [13]. Their electrochemical behaviour and in particular, the CO2 fixation under mild conditions have been examined in the presence of several nickel(II) complexes used as catalyst precursors. Ni(II) derivatives associated to polydentate nitrogen ligands have been reported as efficient catalysts in the electrochemical carboxylation of atkynes [14] and diynes... 

C. K. Jorgensen, Comparative crystal field studies. II. Nickel(II) and copper(II) complexes with polydentate ligands and the behaviour of the residual places... 

Oxidative addition reactions offer an alternative pathway for the synthesis of nickel cyclometallated complexes. Many derivatives with NGN or PGP pincer ligands have been prepared from the corresponding halogenated precursors and Ni(cod)2. These intramolecular oxidative addition reactions take place under very mild conditions and are usually preferred over cyclometallation when the suitable precursors are available. The formation of small amounts of Ni(iii) byproducts is not rare in the case of the NGN ligands. The synthesis of cyclometallated complexes containing bidentate n c1 51,107,108 q pi3,230 polydentate ligands by oxidative... 

---