Cobalt coordination numbers

Ceitain acid dyes can have thek fastness piopeities impioved by combining the dye with a metal atom (chelation). The most common metal is chromium, although cobalt is sometimes used, and this can be introduced in a number of ways. The basic mechanism is donation of electron pans by groups in the dye (ligands) to a metal ion. For example, has a coordination number of 6, and therefore will accept six lone pans of electrons. Typical ligand groups... 

Despite the weak basicity of isoxazoles, complexes of the parent methyl and phenyl derivatives with numerous metal ions such as copper, zinc, cobalt, etc. have been described (79AHC(25) 147). Many transition metal cations form complexes with Imidazoles the coordination number is four to six (70AHC(12)103). The chemistry of pyrazole complexes has been especially well studied and coordination compounds are known with thlazoles and 1,2,4-triazoles. Tetrazole anions also form good ligands for heavy metals (77AHC(21)323). 

Metal ion complexation rates have been studied by the T-jump method. ° Divalent nickel and cobalt have coordination numbers of 6, so they can form complexes ML with monodentate ligands L with n = 1—6 or with bidentate ligands, n = 1-3. The ligands are Bronsted bases, and only the conjugate base form undergoes coordination with the metal ion. The complex formation reaction is then... 

A simplified reaction scheme is shown in Fig. 26.5 Again, the ability of rhodium to change its coordination number and oxidation state is crucial, and this catalyst has the great advantage over the conventional cobalt carbonyl catalyst that it operates efficiently at much lower temperatures and pressures and produces straight-chain as opposed to branched-chain products. 

Mapsi et al. [16] reported the use of a potentiometric method for the determination of the stability constants of miconazole complexes with iron(II), iron(III), cobalt(II), nickel(II), copper(II), and zinc(II) ions. The interaction of miconazole with the ions was determined potentiometrically in methanol-water (90 10) at an ionic force of 0.16 and at 20 °C. The coordination number of iron, cobalt, and nickel was 6 copper and zinc show a coordination number of 4. The values of the respected log jSn of these complexes were calculated by an improved Scatchard (1949) method and they are in agreement with the Irving-Williams (1953) series of Fe2+ < Co2+ < Ni2 < Cu2+ < Zn2+. 

The data analysis in Table 9.3 summarizes the crystallographic information of the Co-Mo-S phase active for hydrodesulfurization. The Co-S distance in Co-Mo-S is 0.22 nm, with a high sulfur coordination of 6.2 1.3. Each cobalt has on average 1.7 0.35 molybdenum neighbors at a distance of 0.28 nm. Based on these distances and coordination numbers one can test structure models for the CoMoS phase. The data are in full agreement with a structure in which cobalt is on the edge of a MoS2 particle, in the same plane as molybdenum. 

The EXAFS data recorded after exposure to air of the unsupported Co-Mo catalysts with different cobalt content allow one to examine the effect of cobalt. In spite of a great uncertainty in the coordination numbers, the promoted catalysts seem to have a somewhat smaller domain size than the unpromoted catalyst as indicated both by the smaller second shell coordination numbers and by the larger effect of air exposure (i.e., reduced sulfur coordination number in first shell). This influence of cobalt on the domain size may be related to the possibility that cobalt atoms located at edges of M0S2 stabilize the domains towards growth in the basal plane direction. Recent results on C0-M0/AI2O3 catalysts indicate that Co may also have a similar stabilizing effect in supported catalysts (36). 

Cobalt in its trivalent state forms many stable complexes in solution. In these complexes, the coordination number of Co + is six. The Co2+ ion also forms complexes where the coordination number is four. Several complexes of both the trivalent and divalent ions with ammonia, amines, ethylene diamine, cyanide, halogens and sulfur ligands are known (see also Cobalt Complexes). 

Cobalt forms many complexes in both the divalent and trivalent states. While the d Co2+ ion exhibits a coordination number of four or six in the trivalent state, the d Co3 ion mostly exhibits coordination number six. Also, trivalent cobalt forms more stable complexes than Co2+ ion, and there are many more of them. The most common donor atom in cobalt complexes is nitrogen. 

Vitamin Bjj (8.50, cobalamin) is an extremely complex molecule consisting of a corrin ring system similar to heme. The central metal atom is cobalt, coordinated with a ribofuranosyl-dimethylbenzimidazole. Vitamin Bjj occurs in liver, but is also produced by many bacteria and is therefore obtained commercially by fermentation. The vitamin is a catalyst for the rearrangement of methylmalonyl-CoA to the succinyl derivative in the degradation of some amino acids and the oxidation of fatty acids with an odd number of carbon atoms. It is also necessary for the methylation of homocysteine to methionine. 

X-ray absorption near edge structure (XANES) is useful in determining the coordination number and the oxidation state of metal ions (Sankar et al, 1983). In Figs. 2.16 and 2.17 we show the XANES of Co and Cu in some compounds as well as catalysts. The ls-3[Pg.99]

What is the coordination number of cobalt and nickel in the am-mines What type of complexes (strong-held or weak-held) do they belong to What type of orbital hybridization occurs in the formation of these complexes What spatial conhguration does this correspond to ... 

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