Ni III Complexes

The prepared Ni(Ill) complex, like other Ni(lII) complexes, has a coorination number of 4. Its structure could be either square planar or tetrahedral. Its d configuration indicates that it will have 1 unpaired electron if it is planar but 3 if it is high spin tetrahedral. Magnetic measurements are important for establishing its structure. Tetrahedral d complexes may have 3 spin-allowed d-d transitions diough some may be in the near ir region. The ir spectrum is likely to be ascribed to the ligand. 

Grind your prepared complex into a fine dry powder and pack it into the tube of a Gouy or Evans balance and measure its magnetic susceptibility and hence its effective magnetic moment and suggest its structure. 

Prepare 0.2 M solution of your complex in 0.1 M KOH and run its spectrum over the range 700-400 nm. Dilute in stages using KOH solution to obtain 10 M solution and run its u.v. spectrum. Draw your conclusions. 

Make a KBr disc of biuret and of your preparation and run their ir spectra. Assign the bands and compare the two spectra. 

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Only the Ni(III) complexes display epr signals. Those or Ni show a splitting of the gj feature owing to the nuclear spin (I = 3/2) of Ni. This is missing in the corresponding Ni complexes. 

Complexes of Ni(III) are synthesized by the oxidation of Ni(II) analogues in aqueous or nonaqueous media. Electrochemical oxidation (51, 105,106), chemical oxidation using (NH4)2S208, cone. HN03, NOBF4, or Co(H20)e+, and pulse radiolysis (107-110) are the most commonly used methods. Ni(III) complexes can be prepared from Ni(II) complexes when the Ni(II)/Ni(III) oxidation potentials are less than +1.1 V vs SCE. For instance, the Ni(III) species can be prepared from [Ni (cyclam)]2+ and l-3a, but cannot be obtained from the polycyclic complexes (9-14), which have Ni(II)/Ni(III) oxidation potentials in the range +1.25-+ 1.50 V vs SCE. 

Some other Ni(III) complexes containing saturated macrocyclic ligands are reported to decompose to Ni(II) complexes of oxidized macrocycles in alkaline solution of 9.0 < pH < 11.0. The reaction involves the Ni(III) complex of the deprotonated ligand (119). 

Ni(III) complexes often exhibit equilibrium with Ni(II) ligand radical species. For example, the Ni(III) complex of lacunar cyclidene is octahedral [NimL(CH3CN)2]3+ at low temperature, but at high temperature it transforms into the Ni(II) complex with an oxidized ligand radical [NinL+]3+, identifiable by the absorption at 590 nm. The Ni(III) complexes and Ni(II) species with an oxidized ligand radical exhibit a thermal equilibrium (Eq. 12) (105). 

The Ni(III) complexes can be isolated as solids, and some of their X-ray crystal structures have been determined (113, 120, 120a). The Ni(III) complexes are usually found to have an octahedral geometry with axially elongated bonds, and the square-planar complex is extremely rare. The X-ray structural parameters for some Ni(III) complexes are summarized in Table VI. The average Ni—N bond distances of octahedral Ni(III) complexes are ca. 0.1 A longer than those of the... 

Ni(III) complexes oxidize some substrates. For example, [Ni(cy-clam)]3+ oxidizes hydroquinone and catechol in aqueous perchlorate media. The kinetics of the reactions have been studied (122). 

The Ni(III) complex of Lie oxidizes the cyclohexanone in MeCN containing CF3C02H (123). 

With primary halides, dimers (R—R) are formed predominantly, while with tertiary halides, the disproportionation products (RH, R(—H)) prevail. Both alkyl nickel(III) complexes, formed by electrochemical reduction of the nickel(II) complex in presence of alkyl halides, are able to undergo insertion reactions with added activated olefins. Thus, Michael adducts are the final products. The Ni(salen)-complex yields the Michael products via the radical pathway regenerating the original Ni(II)-complex and hence the reaction is catalytic. In contrast to that, the Ni(III)-complex formed after insertion of the activated olefin into the alkyl-nickel bond of the [RNi" X(teta)] -complex is relatively stable. Thus, further reduction leads to the Michael products and an electroinactive Ni"(teta)-species. 

The reaction of [Ni(n-Bu2Dtc)3] Br with Ni(n-Bu2 Dtc)2 results in a solution with an EPR spectrum that has been attributed to a reactive asymmetric Ni(III) complex (231). This complex may be similar to that reported by Willemse et al. (631), who obtained Ni(IIIX -Bu2Dtc)21 by the low-temperature (-30°C) oxidation of Ni(n-Bu2Dtc)2 with I2. This complex is characterized by a C—N stretching frequency of 1518 cm-1 and a M—S vibration at 377 cm 1. The /ueff of 1.33 BM is low for a five-coordinate or six-coordinate low-spin complex. Axial EPR spectrum was observed with = 2.260, g2 = 2.215, andg3 = 2.027. 

Ni(III) complexes, 8, 118—119 preparation, 8, 63 via transmetallation, 8, 69—70 Nickel-porphyrin complexes, with platinum(II), 8, 477 (j-Nickel precursors... 

The oxidation of [Ni(mn<)2] to [Ni(mn02] to give Ni(III) complexes has been described 25, 63). However, alternative formulations of these complexes as species containing nickel (II) and radicals have also been proposed 86, 87, 88, 90). The best description of complexes of nickel, where the metal atom has a formal oxidation state different from Ni(II), is still being discussed. The extensive review of these sulfur donors and a... 

Oxidative cleavage can occur with some radicals, such as when H02 is oxidized to 02 + H +. Oxidative cleavage of H02 is actually a form of proton-coupled electron transfer (PCET), discussed below. It occurs in the reaction of H02 with Cu2 +, 32 Ce4 +, Am4+, various Ni(III) complexes, and [Ru(bpy)3]3 +, 19... 

Reductive nitrosylation, on the other hand, can refer to the addition of NO to a metal center Mox with formal reduction of the metal center to yield Mred(NO +), but in the context of ligand reactions reductive nitrosylation refers to the net reactions of NO with metal-bound NO and the ensuing events. Reductive nitrosation of coordinated amines to form nitrosamines occurs through the conjugate base of the amine, and this process has been reported for reactions of NO with [Ni(tacn)2]3 +, 198 with methyl-amine coordinated to a macrocyclic Ni(III) complex,199 with triglycyl complexes of Fe(III), Ni(III), and Cu(III),200 and with Cu(II) macrocyclic complexes.201 Reductive nitrosation of [Ru(NH3)6]3+ produces [Ru(NH3)5N2]2 + with base-catalyzed kinetics the coordinated N2 is produced by hydrolysis after the nitrosation step.170... 

Oxidation of the above complexes to the corresponding Ni(III) species was found to occur readily using either chemical or electrochemical means (under acidic conditions). EPR spectral data for the bis(macrocyclic) Ni(III) complexes are very similar and confirm the presence of a d electronic configuration in which the metal is in a tetragonally distorted environment. 

Bhattacharya et al. [86BHA/MUK] synthesised the tris(2,2 -bipyridine 1,1 -dioxide) Ni(III) complex, Ni(bpyOj)3, and measured the potential of Reaction (V.21) in acetonitrile against the standard calomel electrode (SCE). 

This was attributed to the presence of one electron in the antibonding e orbital in the Ni(III) complex. ... 

It has been reported that the trans-dichloro Ni(III) complex, L2i)Cl2](Cl)(C104)2 2H2O, was isolated during the crystallization of the Ni(II) complex of H2L i in acidic perchlorate media containing chloride 117). The Ni(III) species shows an axial epr spectrum with = 2.17 and ii = 2.02, and A = 30 G. This complex has a pseudooctahe-dral geometry with an average Ni(III)—N bond distance of 1.893(14) A and Ni—Cl bond distance of 2.415(60) A. 

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