We report a novel example of supramolecular cages containing a Lewis acidic trigonal boron center. Self-assembly of the tris(pyridyl)borane donor 1 with diruthenium (2) or platinum (3), as an electron acceptor, furnished boron- a containing trigonal prismatic supramolecular cages 5 and 6, which were characterized by H-1 NMR and electrospray ionization time-of-flight mass spectroscopy and X-ray crystallography. The molecular structure of cage 5 was confirmed as a trigonal prismatic cage with an inner dimension of about 400 angstrom(3). The fluoride binding properties of borane ligand 1 and Pt cage 6 were studied. UV/vis absorption titration studies demonstrated that the boron center of cage 6 undergoes strong binding interaction with the fluoride ion, with an estimated binding constant of 1.3 X 10(10) M-2 in acetone based on the 1:2 binding isotherm. The binding was also confirmed by H-1 NMR titration. Photoluminescence titration studies showed that cage 6 emitted borane-centered fluorescence (tau = 2.21 ns), which was gradually quenched upon addition of fluoride. When excess fluoride was added to a solution of 6, however, dissociation of the pyridyl ligand from the Pt(II) center was observed.