A useful rule, devised by Linus Pauling, is that the most stable arrangement of ligands about central atom is for the atom with highest electronegativity to have the lowest electronegativity ligands, and vice versa. This relationship is seen in many ligand exchange reactions. For more information look in the Chemogenesis
webbook sections on electronegativity,
reactions and substitution,
transfer, abstraction, displacement (STAD).