Copper aquo ion

In contrast, investigation of the effect of ligands on the endo-exo selectivity of the Diels-Alder reaction of 3.8c with 3.9 demonstrated that this selectivity is not significantly influenced by the presence of ligands. The effects of ethylenediamine, 2,2 -bipyridine, 1,10-phenanthroline, glycine, L-tryptophan and L-abrine have been studied. The endo-exo ratio observed for the copper(II)-catalysed reaction in the presence of these ligands never deviated more than 2% from the endo-exo ratio of 93-7 obtained for catalysis by copper aquo ion. 

Figure 3.5. Gibbs energies of complexation of 3.8a-g to the copper(II)(Lr tryptophan) complex versus those for complexation to copper aquo ion.

When a copper(II) salt dissolves in water, the complex aquo-ion [Cu(H2p)6P is formed this has a distorted octahedral (tetragonal) structure, with four near water molecules in a square plane around the copper and two far water molecules, one above and one below this plane. Addition of excess ammonia replaces only the four planar water molecules, to give the deep blue complex [Cu(NH3)4(H20)2] (often written as [Cu(NHj)4] for simplicity). TTo obtain [Cu(NH3)6], water must be absent, and an anhydrous copper(II) salt must be treated with liquid ammonia. 

The effects of these ligands on the second-order rate constants for the Cu (ligand) catalysed reaction of Ic with 2 are modest In contrast, the effects on IC2 are more pronounced. The aliphatic Oramino acids induce an approximately two-fold reduction of Iv relative to for the Cu" aquo ion. For the square planar coordinated copper ions this effect is expected on the basis of statistics. The bidentate ligands block half the sites on the copper centre. 

The systems with which we have been working involve aquo-copper(II) ion reacting with a series of cyclic and open-chain polythioether ligands, each with four or more donor atoms. We first noticed that, as one increases the concentration of "inert" anions, such as CIO, BF, and CF SO, the stability of the... 

Figure 3. Rough ratios of equilibrium concentrations for inorganic (31.321 and organic (20-221 copper species to those of the aquo ion Cu2+, with sulfide complexes plotted as functions of free bisulfide ion level and set at the lower limit in Table II.

When hydrated crystals are heated the water molecules may be lost in stages. For example, copper(II) sulfate pentahydrate changes to the monohydrate (CUSO4.H2O) at 100°C, and to the anhydrous salt (CUSO4) at 250°C. The water molecules in crystalline hydrates may be held by hydrogen bonds (as in CUSO4.H2O) or, alternatively, may be coordinated to the metal ion as a complex aquo ion. 

Fig. 2. Procedure to draw potential-pH diagram at pH 9 and 298 K in a CuO aqueous suspension. Vertical line indicates the equilibrium relationship of an acid-base reaction and horizontal line indicates that of a redox reaction. Diagonal line indicates an equilibrium relationship of the reaction including both an acid-base reaction and a redox reaction. In this procedure, the equUibrium activities of Cu2+ aquo ion was set to be 7.5 x lO-ii considering the following copper species Cu2+, CuO, CU2O, and Cu. Oxyanions of copper, HCuQz" and Cu022", or hydroxyanions of copper, Cu(OH)3 and Cu(OH)42- (Beverskog, 1997), are not considered for simpUcity. In order to include such ions, one should consider all the additional possible equihbrium relationships.

Some aquo cations may be adsorbed non-specifically. This is the case for Cu in an acid medium (pH<3) impregnated in porous silica, where the pores, axe small (<6nm). The EPR spectrum of copper is characteristic of hexa-aquo ions [Cu(OH2)g] exhibiting axial symmetry [6], but their rotation and reorientation... 

There is no evidence that reaction (61) takes place at a measurable rate under ambient conditions, but it is rapid in the presence of aquo-complexes and some organo-complexes of copper ions via reactions (62) and (63) ... 

The coordination number of divalent copper in its complexes is almost always four. The aquo- and halo- complexes have been mentioned, and the reader should recall the blue-purple Cu(NH3)l+ complex which may have been introduced to him as his first example of a complex ion. The Cu(OH)4 complex is only moderately stable, and divalent copper is thus very slightly amphoteric. The four bonds attached to copper in these complexes generally point to the corners of a square. 

This leads us to the discussion of the methods by which the throwing power can be increased. Increasing the conductivity of the solution is an obvious approach, but is limited in scope. A specific resistivity of 5 ii cm, found it the case of the so-called acid copper bath, which contains CuSO and H SO, is about as low as one can get in aqueous solutions. The other approach is to decrease i. The kinetics of metal deposition from tlie simple ions is usually fast, but when the ion is complexed, much lower values of the exchange current density can be realized. This is one of the reasons for using cyanide baths for the electrodeposition of many metals. Copper, for example, can be deposited from an alkaline bath containing KCN. Instead of the usual aquo-complex [Cu(H O) ] one has the much more stable (K = 5.6x10 ) cyanide... 

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