Why does Mercurous ion peculiarly exist as a dimer instead of monomer like a Mercuric ion?

After years of speculation of the nature of the mercury (I) ion, I've come to the following conclusion. Mercury (I) is a dimer consisting of a mercury (II) ion and a neutral mercury atom. There are no individual mercury (I) ions. This is why there is no HgCl, only Hg2Cl2. It is said that mercury (I) (Hg2^2+) disproportionates to form mercury (II) and mercury metal, as if this were a redox reaction and the mercury was changing oxidation state. (Similar to the disproportionation of Cu(I) to form Cu and Cu(II).) But instead of a redox reaction to make Hg and Hg(II) from Hg2^2+, it simply decomposes. There is no electron transfer, because the Hg2^2+ ion exists as at atom of mercury bonded to a Hg2+ ion, as in Hg-Hg2+. There are many examples of the decomposition of mercury (I) compounds to form mercury (II) compounds and metallic mercury. The classic example is in the Group One qualitative analysis test for mercury, where NH3 is added to Hg2Cl2 and it forms mercury metal and HgNH2Cl, where Hg is +2. read less