The present invention is generally directed to a method of prophylaxis against viral infection of a cell or subject or a method of treating a subject infected with a virus including administering an antiviral composition having the general Structure III, wherein each of R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5 and R.sub.6 is the same or different and includes an N-based ligand donor atom selected from the group consisting of ammonia, primary amine or secondary amine, or salt thereof. The present invention is also generally directed to a method of preparing an antiviral agent including providing a cobalt pentammine salt having a non-amine coordination site and mono-substituting the non-amine coordination site with a functional group incorporating a strong coordinator atom to cobalt to form a CoHex structure of Structure III, in which R.sub.1 incorporates the functional group having the strong coordinator atom coordinated with the cobalt atom, or a salt thereof.