Final answer:
Reheating is required in the synthesis of [Co(NH₃5NO₂]Cl₂ because the coordination of NO₂- as a nitro ligand is an endothermic process that requires heat to proceed. In contrast, [Co(NH₃)₅ONO]Cl₂ does not require reheating as its formation may involve lower activation energy or be exothermic.
Step-by-step explanation:
The synthesis of complexes like [Co(NH₃)₅NO₂]Cl₂ and [Co(NH₃)₅ONO]Cl₂ involves the coordination of ligands to a metal center. The ligand NO₂ can bind to cobalt in two different ways, either through the nitrogen atom as nitro (NO₂-) or through the oxygen atom as nitrito (ONO-). The formation of the nitro complex is generally associated with an endothermic reaction, requiring an input of heat to encourage the reaction to proceed. This is due to the stronger bonding that occurs when NO₂ coordinates through nitrogen. Conversely, the nitrito complex typically requires no additional heating due to the reaction being sufficiently exothermic or having lower activation energy.
Reheating is necessary for the synthesis of the nitro complex because heat acts as a reactant and helps to overcome activation energy barriers, allowing the reaction to reach completion. The enthalpy change of the reaction is considered when determining whether an external heat source is needed. In the case of [Co(NH₃)₅NO₂]Cl₂, the enthalpy change is such that additional heat is required to drive the formation of the complex.