# Why is the probability of a didentate ligand to form a chelate with a central atom, high?

When one ligating group of didentate ligands (like ethylenediamine) is coordinated with a central atom, then the probability of the other ligating group being coordinated, and hence forming a chelate, is high. This is seems to be because one of the ligating groups is bound to be closer to the central atom when the other is coordinated. Does this always hold true? If so, why?