As part of our ongoing investigations in the chemistry of main group Lewis acids, we have recently become interested in the properties of heavy pnictogenium and chalcogenium ions. Owing to the electropositive character of the central atom as well as to the presence of low lying vacant orbitals, these onium ions behave as unusual Lewis acids. In this presentation, we will illustrate some of these characteristics by describing how derivatives containing phosphonium, stibonium or telluronium ions can be used for the selective complexation of fluoride ions in protic solvents. We will also report that some these compounds can be used to harvest 18F fluoride ions in water for applications in Positron Emission Tomography. In the last part of this seminar, we will show that heavy pnictogens and chalcogens can behave as Z-ligands and engage electron rich transition metals in unusual donor acceptor interactions.