Complexes of Tripositive Cobalt

A number of such complexes may be made by oxidizing a solution containing divalent cobalt, together with the appropriate ligand. Other complexes are most easily prepared by substitution of ligands on a complex already formed. The following is a set of typical conversions  

Since there are a number of complexes that might conceivably be formed from most combinations of reagents (Exercise 8), the obtaining of a desired complex in substantial yield is in many cases an art. In identifying coordination compounds of cobalt, halogen atoms held in the complex ion are distinguished from anionic halides by treatment with Ag+. This precipitates anionic chloride, bromide, and iodide immediately, but eom-plexed halide precipitates only slowly. Thus, with the purpose compound 

As is the case with Fe(III) compounds, the colors of the familiar Ni2+ solutions used as laboratory reagents are not representative of the colors of the salts themselves. The majority of anhydrous nickel salts are yellow but as with copper(II), the color may deepen markedly as the polarizability of the anion increases. Thus NiBr2 is yellow, Ni(SCN)2 brown, and Nil2 black. The green color of aqueous nickel solutions is, of course, that associated with the ion, Ni(H20) +. Although divalent nickel may, in the presence of suitable complexing agents be oxidized to the +3 and +4 or reduced to the +1 state, such reactions are rarely carried out, and the +2 state is by far the most important. 

Ni(II) forms an interesting variety of complexes. Some of the most familiar are octahedral (for example, Nl(H20)6 2, green Ni(NH3)g2, blue, and Ni(en) 2, violet), a few are tetrahedral (for example, (Et3P—)2NiCl2, 

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