Molecular capsules consist of closed, hollow frameworks within which encapsulated molecules are isolated from interaction with external molecules .In this environment, otherwise reactive molecules can be stabilized [2-5]. Although some molecular capsules have been prepared by conventional synthetic chemistry , recent progress in non-covalent synthesis has allowed the creation of capsules held together by hydrogen bonds [6-9]. Here we report the use of transition-metal-based coordination chemistry [10-19] to assemble a stable, nanometre-scale capsule from 24 small components: 18 metal ions and six triangular ligands. The capsule is roughly hexahedral and comprises six edge-sharing triangles with two metal ions on each edge. The internal space has a volume of 900 Angstrom  and is fully closed to all but very small molecules.