The second metal complex is much less likely to lose one of the ligands due to the bridging group that holds the ligands in proximity to the metal center. From a kinetic viewpoint, if one of the ligands dissociates, it will remain close enough to the metal center to have a high probability of re-coordinating before another ligand can get in an bind. From a thermodynamic viewpoint, by tethering two donor ligands together, one removes the entropic driving force for dissociating a ligand and thus making more particles in solution (more disorder). The chelate effect can be extremely dramatic. There are cases known where the presence of a chelate will change the equilibrium constant by a factor of 1010 favoring the coordinated form of the ligand to the metal over the dissociated form. Naturally, the longer and more flexible the bridging group in a chelating ligand, the less dramatic the chelating effect. It is possible to design ligands with sterically enforced chelates. Some chelating ligand examples (line drawings and crystal structure shown in style; atom coloring: carbon = grey, oxygen = red, “tube” nitrogen = light blue; sulfur = yellow; metal = magenta):
The second metal complex is much less likely to lose one of the ligands due to the bridging group that holds the ligands in proximity to the metal center. From a kinetic viewpoint, if one of the ligands dissociates, it will remain close enough to the metal center to have a high probability of re-coordinating before another ligand can get in an bind. From a thermodynamic viewpoint, by tethering two donor ligands together, one removes the entropic driving force for dissociating a ligand and thus making more particles in solution (more disorder). The chelate effect can be extremely dramatic. There are cases known where the presence of a chelate will change the equilibrium constant by a factor of 1010 favoring the coordinated form of the ligand to the metal over the dissociated form. Naturally, the longer and more flexible the bridging group in a chelating ligand, the less dramatic the chelating effect. It is possible to design ligands with sterically enforced chelates. Some chelating ligand examples (line drawings and crystal structure shown in “tube” style; atom coloring: carbon = grey, oxygen = red, nitrogen = light blue; sulfur = yellow; metal = magenta):