Lower oxidation states

The +1 oxidation state is quite stable for mercury, and always involves the dimeric [Hg-Hg]²⁺ ion. Proof for this comes from solid-state structures and in solution from several sources:

1. Hg^I species are diamagnetic where Hg⁺ would have an unpaired electron;
2. Raman spectra of solutions depict a band from the Hg-Hg stretching vibration identical to that seen in solids;
3. Equilibrium studies (example through electrochemistry) are consistent with

with an equilibrium constant [Hg²⁺]/[ Hg²⁺₂]at 25°C; the equilibrium expression including Hg⁺ would have a dissimilar form.

In aqueous solution Uncomplexed Hg²⁺₂ is marginally stable, but the disproportionation equilibrium can be upset through any ligand for which the Hg^II compound is more stable. So addition of sulfide, cyanide and several other ligands causes disproportionation. In solid compounds the  Hg²⁺₂ ion all the time has two ligands strongly bonded. For instance, Hg₂Cl₂ has linear Cl-Hg-Hg-Cl molecules and salts with noncomplexing anions like nitrate consist the hydrated ion [H₂O-Hg-Hg-H₂O]²⁺.

Oxidation of Hg with AsF₅ gives species consisting of linear Hg²⁺₃and Hg²⁺₄ions, culminating in a metallic compound Hg_0.33AsF₆, that consists of linear chains of mercury atoms. Zn and Cd analogs of Hg²⁺₂are much less stable, mainly due to the larger lattice energies acquired with the smaller M²⁺ ions tend to force disproportionation. Zn²⁺₂ and Cd²⁺₂ can both be recognized spectroscopically while the elements react with melts of the subsequent chloride. Adding AlCl₃ provides the solid compound [Cd²⁺₂] [AlCl₄^-]₂but no solid zinc (I) compounds have been ready.