The stella octangula cage [Pd₆L₈]¹²⁺

Assemble eight tripodal ligands and six palladiums, and you get the cage [Pd₆L₈]¹²⁺. Its +12 charge comes from the six Pd(II) ions (+2 each); it is balanced by twelve nitrate anions NO₃⁻ in solution.

Its geometry is remarkable. The six Pd occupy the vertices of an octahedron. Now, an octahedron has exactly eight triangular faces — and there are precisely eight ligands, one per face, each pointing its bowl outward. This spiky shape is a stella octangula, the first stellation of the octahedron: you extend the edges until they meet in points.

          Pd                (6 Pd = vertices of the octahedron)
         /|\               (8 faces = 8 tripodal ligands)
        / | \              (each bowl caps one face)
     Pd--+--Pd
        \ | /              ->  eight-pointed star look
         \|/                   (stella octangula)
          Pd

Two properties follow. First, the cage encloses a large internal volume: it is about 3 nm in diameter and behaves like a nano-container able to host guest molecules. Second, its surface is not closed: it is pierced by eight windows of about 7 × 9 Å, partially blocked by the ligands' methoxy groups. These openings let guests in and out.

The appeal of the approach is that it is tunable. By lengthening or stiffening the ligand's arms, you enlarge the cage and adjust the size of its windows, without changing the assembly principle. You thus get a whole family of tunable super-hosts — exactly the goal of the study's authors, who tested seven different ligands to map out what can be built.