Hey,

The leaving group tendency is based on the aside base strength. The basic compound whose conjugate acid will will be stronger than water will act as a good leaving group.

On the other hand the Nucleophilic property is based on the bond between atom and it's line pair of electrons in the outermost shell.

So a good nucleophile can be a good leaving group.

Iodine is the best example of it. The conjugate acid of Iodine HI is stronger than water hence it act as a good leaving group. Along with tha the outer most lone pair of electron is loosely bounded hence its easy to give up the electron and act as a good nucleophile.

Hope that helps.

Thank you.