Applications in metal remediation, both environmental and medical, are envisioned by designing the ligands to target a specific toxic or hazardous metal ion. Here we are elaborating a design strategy for forming coordination capsules comprised of toxic metal ions or main group elements (such as arsenic, antimony and bismuth). We have shown that ligand H2L binds arsenic(III) within a very stable As2L3 cage. Surprisingly, two As2L2Cl2 macrocycles were also isolated as stable, kinetic intermediates in the reaction. Due to the dynamic covalent nature of the pnictogen-thiolate bond, these Group 15 capsules are observed to be highly reactive, allowing for transmetalation, conformational isomerism, heterometallic self assembly and interesting secondary bonding interactions. Furthermore, the novel cavity environments in these capsules will lead to unusual host-guest properties.