What is the formation constant denoted as, and what does it measure?

The formation constant, commonly denoted as Kf, is a crucial parameter in coordination chemistry that measures the stability of a complex formed between a metal ion and a ligand. A high formation constant indicates that the complex is stable, meaning that once the ligand binds to the metal, it is less likely to dissociate back into free metal ions and ligands. This stability is key in understanding the behavior of metal-ligand complexes in various chemical and biological processes.

In contrast, the other options refer to different constants or measurements that address different concepts. Kd, for instance, represents the dissociation constant and measures how easily a complex can dissociate back into its components, and is not associated specifically with the stability of ligand binding. Ksp, or the solubility product constant, relates to the solubility of ionic compounds in solution rather than coordination chemistry. Lastly, pH is a measure of acidity and has no direct relation to the formation constants of metal-ligand complexes. Thus, the connection of Kf to the stability of ligand binding makes it the correct choice in this context.