Complex sentence: The perrhenate anion is also used as a ligand that coordinates less strongly than Cl-or Br-but stronger than ClO 4-or BF 4 -.
Split into simpler sentences: The perrhenate anion is also used as a weakly coordinating anion. It is a weaker base than Cl-or Br-but stronger than ClO 4-or BF 4 -.