Inorganic compounds have been used in medicine for many centuries but often only in an empirical fashion with little attempt to design the compound used and with little or no understanding of molecular basis of their mechanisms of action. There is enormous scope for the design of novel therapeutic and diagnostic metal compounds. The toxicity of an element will depend on the element itself, its oxidation state, and the nature and number of coordinated ligands, as well as on the dose, mode of administration, and biochemical states of the host. Moreover, the effect of one element may depend on the presence or availability of another. It is seldom useful to describe elements as "toxic" or "non-toxic". Even so-called "toxic compounds" can usually be tolerated in low doses, and may exhibit therapeutic effects within narrow concentration ranges, and biochemically-essential elements can become toxic at high dose (1). The Bertrand Diagram,