The most common ones are with mercury(I), which is Hg22+.
It is NEVER Hg+ by itself. It ALWAYS comes as a pair. There are reasons for this behavior, but that is beyond the scope of this work. Note: if anybody has found an explanation on the Web, let me know at ChemTeam@ClubNet.net, so I can set up a link. Thanks. And now, back to chemistry.
Example #1: mercury(I) chloride
The formula for this compound is Hg2Cl2. It is not HgCl. You will be marked wrong on a test for doing so.
Example #2: mercury(I) nitrate
The formula for this compound is Hg2(NO3)2. Once again, it is not reduced. Why? In nature, mercury(I) comes in a set of two atoms, NOT just one.
The corresponding mercury(II) formula for the two examples would be HgCl2 and Hg(NO3)2.
The second major category is peroxide, which is O22¯.
Example #3: hydrogen peroxide
The formula is H2O2 and it is not reduced to HO. Why? Same reason as above, peroxide travels as a group of two oxygen atoms, not one.
Example #4: sodium peroxide
The formula is Na2O2.