Cobalt coordination compound solution

Procedure 2.3.b Preparation of the Cobalt Coordination Compound Sample Solution—Decomposition and Reduction of Cobalt(lll) to Cobalt(ll). 

Potassium Hexanitrocobaltate(III). Add a little acetic acid and an excess of potassium nitrite to a cobalt(II) salt solution. Heat the mixture. What gas evolves What precipitates Write the equation of the reaction. What is the coordination number of cobalt in this compound ... 

Kekule s instability criterion failed completely in the case of many coordination compounds, which were classified as molecular compounds by sheer dint of necessity although they were extremely resistant to heat and chemical reagents. For example, although hexaamminecobalt(III) chloride contains ammonia, it neither evolves this ammonia on mild heating nor does it react with acids to form ammonium salts. Also, addition of a base to its aqueous solution fails to precipitate hydrated cobalt(III) hydroxide. 

In their first publication on this subject,59 Werner and Miolati showed that the molecular conductances (fx) of coordination compounds decreased as successive molecules of ammonia were replaced by acid residues (negative groups or anions). For cobalt(III) salts, they found that fi for luteo salts (hexaammines) > fi for purpureo salts (acidopentaammines) > /t for praseo salts (di-acidotetraammines). The conductance fell almost to zero for the triacidotriammine Co(N02)3-(NH3)3 and then rose again for tetracidodiammines, in which the complex behaved as an anion. By such measurements, Werner and Miolati determined the number of ions in complexes of cobalt(III), platinum(II) and platinum(IV). They not only found support for the coordination theory, but they also elucidated the process of dissociation of salts in aqueous solution and followed the progress of aquations. 

Evidence forcoball(l) was first obtained from the electrolytic reduclion of cyano-compounds and some of the reduced species have been isolated. There arc also many cohaltfl) coordination compounds of Ihe organomelallic class carbonyl, isonitriles, and unsaturated hydrocarbon derivatives. The oxidation state cobalt(O) may be represented in the eyano-Lompound which has been formulated as Ks[Cu (CN)b. It has been prepared as an air-sensitive brown-violet compound by reducing a liquid ammonia solution of Kj Co(CN)f,l with an excess of K tnctal. The only other known coball(O) species arc organomelallic compounds. 

The chemistry of cobalt involves mainly its +2 and +3 oxidation states, although compounds containing cobalt in the 0, +1, or +4 oxidation states are known. Aqueous solutions of cobalt(II) salts contain the Co(H20)62+ ion, which has a characteristic rose color. Cobalt forms a wide variety of coordination compounds, many of which will be discussed in later sections of this chapter. Some typical cobalt compounds are listed in Table 20.8. 

A coordination compound of cobalt(III) contains four ammonia molecules, one sulfate ion, and one chloride ion. Addition of aqueous BaCli solution to an aqueous solution of the compound gives no precipitate. Addition of aqueous AgN03 to an aqueous solution of the compound produces a white precipitate. Propose a structure for this coordination compound. 

The tetraamminedinitrocobalt(lll) salts represent the second longest known case of geometric isomerism among coordination compounds. The reddish-yellow trans compounds were first prepared in 1875 by air oxidation of a solution of cobalt(II) chloride containing ammonia, sodium nitrite, and ammonium chloride by Wolcott Gibbs, who called them croceo salts. The brownish-yellow cis compounds were first prepared in 1894 by reaction of tetraamminecarbonato-cobalt(III) salts with sodium nitrite by Jorgensen, who called them flavo salts. The two series correspond completely to the bis(ethylenediamine)dinitrocobalt-(111) salts. 

An improvement of catalyst activity, especially for the oxidation of electron-poor, deactivated systems like p-toluic acid, can be reached by addition of other transition metal compounds to the Co/Mn/Br catalyst. The most prominent additive is zirconium(IV) acetate, which by itself is totally inactive. An addition of zirconi-um(IV) acetate (ca. 15 % of the amount of cobalt) can yield reaction rates which are higher than those observed using a tenfold amount of cobalt acetate. This amazing co-catalytic effect can be attributed to the common ability of zirconium to attain greater than sixfold coordination in solution, to the high stability of Zr toward reduction, and to the ability of zirconium or Hf to redistribute the dimer/ monomer equilibrium of dimerized cobalt acetates (Co 7Co, Co VCo " systems) by forming a weak complex with the catalytically more active monomeric Co species [17]. 

The reaction of urea with cobalt sulfate in aqueous solution gives the coordination compound [Co2(NH2)2(S04)2(H20)2] which is binuclear and includes two bridging sulfate groups [72] ... 

The enthalpies of solution in water of nickel chloride, cobalt chloride and the coordination compounds RbNiCl3, RbCoCl3, Rb2CoCl4 and Rb3CoCl5 were determined in a calorimeter with an isothermal jacket . The authors reported for the reaction. 

The green solution of carbonatocobaltate(III) was prepared from a cobalt(III) coordination compound by McCutcheon and Schule a solution of tetraammine-carbonatocobalt(III) sulfate was added to a hot aqueous solution of potassium bicarbonate and potassium persulfate. The mixture was heated in a steam-bath until its red color changed to dark green. When a hot aqueous solution of hexa-amminecobalt(III) nitrate was added to the above solution, a grayish-green compound precipitated. Its analysis matched the formula [Co(NH3)5][Co(C03)3]. 

When six ligands are attached to a central metal ion, Werner predicted an octahedral structure. Thus, for Co(NH3)5Cl3, he postulated a structure in which the five ammonia molecules and only one of the three chloride ions are ligands tightly bound to cobalt. The other two chloride ions are free in aqueous solution and readily precipitate with Ag. He termed such chemical species coordination compounds and identified six as the coordination number for Co. Further proof came from Werner s identification of cfr-(Cl-Co-Cl angle 90°) and trans- (Cl-Co-Cl angle 180°) isomers for Co(NH3)4Cl3, and the acmal isolation of enantiomers (see the structures depicted) in 1911. For this work, Werner received the Nobel Prize in chemistry in 1913. 

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