Nickel-oxalate complexes

More recently, Rudolph et al. was able to reduce C02 to oxalate with faradaic efficiencies approaching 100% with their most active and stable complex [102]. These authors examined a variety of macrocyclic nickel chelate complexes with various substituent groups on the ring in acetonitrile solution. Whilst it is interesting that the group was able to produce oxalate catalyzed by a metal complex, the potentials required for reduction were —1.9 to —2.2 V (versus SCE), similar to the potential required for the direct reduction of C02 in aprotic solvent (—2.21 V versus SCE). The very negative potentials in this reaction highlight the overall theme of the electrochemical reduction of C02. 

Likewise, the direct synthesis of [M(PF3)4] (M = Ni, Pd, Pt) complexes has been achieved from the appropriate metal powder (method F), or alternatively under very mild conditions from highly reactive forms of the metal (e.g., Ni) generated either from the decomposition of nickel oxalate or nickel tetracarbonyl or activated by sulfide (method G). 

A further possibility of preparing mixed-salt catalysts is provided by decomposition of complex salts of the type A [BXm],181 where A is a non-noble metal cation and B is a noble metal present in a complex anion an example is the oxalate complex Li2[Cu(Ox)2]. In all cases, highly active catalysts result they are usually more efficient than other catalysts containing the metal in question, and maximum activity is found with a molar ratio of, e.g., nickel oxalate magnesium oxalate = 1 1. 

Trifluorophosphine complexes of nickel, palladium, and platinum have been obtained by direct reaction of PF3 with the metal at elevated temperatures and pressures (172). Similarly several fluorophosphine complexes of zero-valent nickel can be made under very mild conditions (60°C) using metallic nickel formed by decarboxylation of nickel oxalate... 

MUR/KUR] Murai, R., Kurakane, K., Sekine, T., The stability constants of nickel(Il) complexes with chloride, thiocyanate, sulfate, thiosulfate and oxalate ions, as determined by a solvent extraction method. Bull. Chem. Soc. Jpn., 49, (1976), 335-336. Cited on pages 146, 148, 234, 377. 

There are now a number of techniques available for the study of fast reactions, and the rate of formation of nickel monooxalate complex has been measured spectrophotometrically at a number of temperatures using a flow technique (9). For this system, all the equilibrium thermodynamic properties have been measured (1), and the kinetic data are consistent with the assumption that the reaction of Ni(II) with oxalate ion in neutral solution occurs in the following steps (9)... 

A reversal of the elution order for metals such as lead, cobalt, zinc, and nickel is obtained with the use of oxalic acid as a single complexing agent. Under these chromatographic conditions, the metal separation is controlled by anion and cation exchange processes. The degree to which each mechanism contributes to the separation process depends on the stability of the oxalate complexes (see Table 4-5) and is, therefore, different for each metal ion. The anion exchange... 

CFg-PFa and iron pentacarbonyl. Metallic nickel (formed by decarboxylation of nickel oxalate) has been shown to react directly at 60 °C with several fluorophosphines, including CFs-PF and (CF8)2PF, to give the corresponding zerovalent nickel complexes, e.g. Ni(CFj PF2)4 analogous complexes of zerovalent platinum, e.g. Pt[(CF3)jPF]4 (which can also be obtained from potassium tetrachloroplatinite and fluorobistrifluoromethylphosphine ), have been prepared from platinum(n) chloride and PFj, CFj-PFj, and (CF3)2PF at 60°C. = ... 

The nickel complex decomposes in three steps ((5.13)-(5.15)). In the first step water is lost endothermically at 124 °C. Thereafter, the anhydrous complex further decomposes to give nickel oxalate monohydrazine. This is observed as an endotherm with a peak temperature of 280 °C. The final step corresponds to the decomposition of nickel oxalate monohydrazine to nickel oxide (NiO). This decomposition is exothermic at 342 °C (Figure 5.7a) ... 

A single-crystal X-ray study of the hydrazinium nickel oxalate dihydrate complex could not be carried out as the crystal shows twinning, rendering the X-ray investigation difficult. Therefore, based upon IR, UV spectra, and magnetic susceptibility data, the proposed structure of hydrazinium nickel oxalate dihydrate is represented as shown in Figure 5.9. In this case, apart from two bidentate (chelate), oxalate groups occupy the equatorial plane, while the axial positions are filled by hydrazinium cations. As a whole the nickel atom displays octahedral coordination. 

Sulphuric acid is not recommended, because sulphate ions have a certain tendency to form complexes with iron(III) ions. Silver, copper, nickel, cobalt, titanium, uranium, molybdenum, mercury (>lgL-1), zinc, cadmium, and bismuth interfere. Mercury(I) and tin(II) salts, if present, should be converted into the mercury(II) and tin(IV) salts, otherwise the colour is destroyed. Phosphates, arsenates, fluorides, oxalates, and tartrates interfere, since they form fairly stable complexes with iron(III) ions the influence of phosphates and arsenates is reduced by the presence of a comparatively high concentration of acid. 

Tinnemans et al.132 have examined the photo(electro)chemical and electrochemical reduction of C02 using some tetraazamacrocyclic Co(II) and Ni(II) complexes as catalysts. CO and H2 were the products. Pearce and Pletcher133 have investigated the mechanism of the reduction of C02 in acetonitrile-water mixtures by using square planar complexes of nickel and cobalt with macrocyclic ligands in solution as catalysts. CO was the reduction product with no significant amounts of either formic or oxalic acids... 

The coordination of oxalate dianion to nickel(II) has been studied in aqueous solution.1773,1774 A number of oxalato complexes including the simple... 

Ni(04C2)(tet-a)-3H20 (tet = 5,7,7,12,14,14-hexamethyl-l,4,8,ll-tetraazacyclotetradecane a = meso isomer b = racemic isomer) were prepared by the reaction of a concentrated solution of sodium oxalate with an aqueous solution of the appropriate nickel(II) amine complex.1780,1781 In the dinuclear complex [Ni2(04C2)(en)4](N03)2 (234)1740-1741 the bridging oxalato group is planar and symmetrically bonded to the two nickel atoms. The same structure occurs in the complex Ni2(04C2)(0N0)2(py)6 which was obtained as a by-product in a very low yield when a pyridine solution of methanenitrosolic acid and nickel(II) were allowed to stand for several months.1741... 

Photochemistry of Oxalate and Dithiooxalate Complexes of Nickel, Palladium, and Platinum... 

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