Nickel complexes five-coordinate

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. 

Most of the nickel(l) complexes are four-coordinate, either tetrahedral (phosphine and arsine complexes) or square planar (macrocyclic and dithiolene complexes), but five-coordinate complexes are also easily formed in the presence of tetradentate tripodal ligands. 

Some nickel(II) tetraaza macrocycles have been proved to act as efficient catalysts for the electrochemical reduction of C02 in H20/MeCN medium. This indirect electroreduction occurs at potentials in the range -1.3 to -1.6 V vs. SCE and mainly produces either CO or a CO/H2 mixture, depending upon the type of complex.2854 The five-coordinate complexes [NiL] (394) formed by some deprotonated dioxopentamine macrocycles have been found to display very low oxidation potentials Nin/Nira in aqueous solution (about 0.24-0.25 V vs. SCE at 25 °C and 0.5 M Na2S04). Air oxidation of the same complexes in aqueous solution yields 1 1 NiL-02 adducts (5 = 1) which are better formulated as superoxo complexes, NimL-02 (Scheme 56). The activation of Ni-bound oxygen is such that it attacks benzene to give phenol.2855... 

Closely related to this tetrahedral nickel molecule is the five-coordinate complex shown in Fig. 15. Bis(7V-/3-diethylamine-ethyl-5-chlorosalisyl-aldiminato)nickel(II) differs from the previous system in three main features AMsopropyl groups are replaced by tertiary amines, the phenyl rings are substituted with chlorine atoms para to the oxygen donor atoms, and the coordination is approximately square pyramidal rather than distorted tetrahedral. Crystal... 

Addition of triphenylphosphine or t-butylisocyanide to 101 affords the analogous five-coordinate complexes 102b and 102c. These species represent rare examples of stable five-coordinate ds-dialkyl complexes of Ni(II), although stable trigonal-bipyramidal trans-dialkyls of nickel are well known (176-178), and they provide a structural model for a putative square-pyramidal intermediate in reductive elimination reactions of ds-dialkyl... 

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-... 

Since the electron density of nickel in bis(7r-crotylnickel iodide) is high, the formation of five-coordinate complexes is favored for compounds with vacant orbitals readily attached by dative bonds (11). The bases might be classed into two types by this characteristic ... 

Iron Pentacarbonyl. In its reactions with liquid dinitrogen tetroxide this compound follows the pattern outlined above, and the remarks on reactions of nickel carbonyl probably apply here also. The solvate, Fe(N03)3.N204, is produced (4). This has the structure N0+[Fe(N03)4] in the solid state it is volatile without decomposition, and is believed to be a five-coordinate complex, Fe(N0)(N03)4, in the vapor state. We have not yet succeeded in isolating the simple trinitrate, Fe(N03)3, by removal of N2O4. 

Factor 430M is diamagnetic in inert solvents, suggesting the presence of a square planar Ni centre. It is paramagnetic when dissolved in a mixture of water and methanol, suggesting that water is coordinated either in both axial positions to give an octahedral complex, or in one position only to give a five-coordinate complex with the nickel out of the plane. 

Sweigart has published three more papers on substitution kinetics of five-coordinate complexes of the [M(dithio-ligand)2L] type. Reactions of the tri-n-butyl-phosphine adducts of bis-(0-dialkyldithiophosphato)nickel(n) complexes with bidentate nitrogen or phosphorus nucleophiles go by the D [5 nl(lim)] mechanism. It now appears that the substitution mechanism operating for this type of complex depends on the charge on the complex, i.e. on the formal oxidation state of the metal. Reactions... 

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