text: Cavitand cages are formed by linking bowl-shaped organic molecules called cavitands. The two "bowls" are linked with organometallic complexes.
  In order for a cavitand cage to efficiently self-assemble, the following requirements must be met: The cavitand scaffold must be rigid, the incoming metal complex must impose cis geometry, and there must be enough preorganization in the structure such that the entropic barrier to create the cage can be overcome. The complexes used to assemble cavitand cages are square planar with one η2 ligand; this helps enforce the final geometry. Without cis geometry, only small oligomers will form. Self-assembly also requires a ligand exchange; weakly bound ions such as BF- and PF- promote assembly because they leave the complex so it can bind with the nitriles on the rest of the structure.