Quimica inorganica

Synthesis and characterization of [Co(NH3)4CO3]NO3 and [Co(NH3)5Cl]Cl2
Introduction: Experiments involving the aqueous preparation of cobalt(III) complexes have been a familiar feature in many textbooks written for use in the inorganic chemistry laboratory.18

Coordination compounds of Co(III) have been of particular interest because their

complexes, kinetically inert, undergo ligand exchange very slowly compared with various other transition metals. The metal complex Ni(NH3)62+ reacts very rapidly with water, undergoing a ligand substitution reaction, to form Ni(H2O)62+. The analogous reactions of Co(NH3)63+ and Cr(NH3)63+ occur very slowly in aqueous solution. Ligand field and molecular orbital theory may be used to qualitatively explain the differences in reactivity.9 The slow reactivities of octahedral complexes have led to extensive investigations of these compounds. In this experiment, you will prepare the coordination compounds [Co(NH3)4CO3]NO3 and [Co(NH3)5Cl]Cl2 via ligand substitution reactions.

Part I: The synthesis of [Co(NH3)4CO3]NO3 involves the following unbalanced equation, Co(NO3)2 + NH3(aq) + (NH4)2CO3 + H2O2 → [Co(NH3)4CO3]NO3 + NH4NO3 + H2O Cobalt nitrate [Co(NO3)2] is deliquescent (tends to absorb atmospheric water vapor). Upon exposure to atmospheric moisture, cobalt nitrate has the formula Co(NO3)2 6H2O. Co(II) complexes react very rapidly by ligand exchange, thus a possible first step in the reaction: Co(OH2)62+ + 4NH3(aq) + CO32- → Co(NH3)4CO3 + 6H2O The intermediate tetraamminecarbonatocobalt(II) could then be oxidized by addition of H2O2 to form [Co(NH3)4CO3]+. Compounds with a carbonato ligand are useful intermediates in the synthesis of coordination complexes. The carbonate ion is easily 1

removed by the additional of HCl and the carbonate forms carbon dioxide. The carbonate ion is a bidentate ligand and leaves two open coordination sites. Water molecules or chloride ions may occupy the open coordination sites. Water is not